Add phone based LF-MMI training.

.flake8

@@ -8,4 +8,6 @@ per-file-ignores =
 
 exclude =
   .git,
-  **/data/**
+  **/data/**,
+  icefall/shared/make_kn_lm.py,
+  egs/librispeech/ASR/conformer_mmi_phone/embedding.py

egs/librispeech/ASR/conformer_mmi_phone/asr_datamodule.py

@@ -0,0 +1 @@
+../tdnn_lstm_ctc/asr_datamodule.py

egs/librispeech/ASR/conformer_mmi_phone/conformer.py

@@ -0,0 +1,933 @@
+#!/usr/bin/env python3
+# Copyright (c)  2021  University of Chinese Academy of Sciences (author: Han Zhu)
+#
+# See ../../../../LICENSE for clarification regarding multiple authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+import math
+import warnings
+from typing import Optional, Tuple
+
+import torch
+from torch import Tensor, nn
+from transformer import Supervisions, Transformer, encoder_padding_mask
+
+
+class Conformer(Transformer):
+    """
+    Args:
+        num_features (int): Number of input features
+        num_classes (int): Number of output classes
+        subsampling_factor (int): subsampling factor of encoder (the convolution layers before transformers)
+        d_model (int): attention dimension
+        nhead (int): number of head
+        dim_feedforward (int): feedforward dimention
+        num_encoder_layers (int): number of encoder layers
+        num_decoder_layers (int): number of decoder layers
+        dropout (float): dropout rate
+        cnn_module_kernel (int): Kernel size of convolution module
+        normalize_before (bool): whether to use layer_norm before the first block.
+        vgg_frontend (bool): whether to use vgg frontend.
+    """
+
+    def __init__(
+        self,
+        num_features: int,
+        num_classes: int,
+        subsampling_factor: int = 4,
+        d_model: int = 256,
+        nhead: int = 4,
+        dim_feedforward: int = 2048,
+        num_encoder_layers: int = 12,
+        num_decoder_layers: int = 6,
+        dropout: float = 0.1,
+        cnn_module_kernel: int = 31,
+        normalize_before: bool = True,
+        vgg_frontend: bool = False,
+        is_espnet_structure: bool = False,
+        mmi_loss: bool = True,
+        use_feat_batchnorm: bool = False,
+    ) -> None:
+        super(Conformer, self).__init__(
+            num_features=num_features,
+            num_classes=num_classes,
+            subsampling_factor=subsampling_factor,
+            d_model=d_model,
+            nhead=nhead,
+            dim_feedforward=dim_feedforward,
+            num_encoder_layers=num_encoder_layers,
+            num_decoder_layers=num_decoder_layers,
+            dropout=dropout,
+            normalize_before=normalize_before,
+            vgg_frontend=vgg_frontend,
+            mmi_loss=mmi_loss,
+            use_feat_batchnorm=use_feat_batchnorm,
+        )
+
+        self.encoder_pos = RelPositionalEncoding(d_model, dropout)
+
+        encoder_layer = ConformerEncoderLayer(
+            d_model,
+            nhead,
+            dim_feedforward,
+            dropout,
+            cnn_module_kernel,
+            normalize_before,
+            is_espnet_structure,
+        )
+        self.encoder = ConformerEncoder(encoder_layer, num_encoder_layers)
+        self.normalize_before = normalize_before
+        self.is_espnet_structure = is_espnet_structure
+        if self.normalize_before and self.is_espnet_structure:
+            self.after_norm = nn.LayerNorm(d_model)
+        else:
+            # Note: TorchScript detects that self.after_norm could be used inside forward()
+            #       and throws an error without this change.
+            self.after_norm = identity
+
+    def run_encoder(
+        self, x: Tensor, supervisions: Optional[Supervisions] = None
+    ) -> Tuple[Tensor, Optional[Tensor]]:
+        """
+        Args:
+          x:
+            The model input. Its shape is [N, T, C].
+          supervisions:
+            Supervision in lhotse format.
+            See https://github.com/lhotse-speech/lhotse/blob/master/lhotse/dataset/speech_recognition.py#L32  # noqa
+            CAUTION: It contains length information, i.e., start and number of
+            frames, before subsampling
+            It is read directly from the batch, without any sorting. It is used
+            to compute encoder padding mask, which is used as memory key padding
+            mask for the decoder.
+
+        Returns:
+            Tensor: Predictor tensor of dimension (input_length, batch_size, d_model).
+            Tensor: Mask tensor of dimension (batch_size, input_length)
+        """
+        x = self.encoder_embed(x)
+        x, pos_emb = self.encoder_pos(x)
+        x = x.permute(1, 0, 2)  # (B, T, F) -> (T, B, F)
+        mask = encoder_padding_mask(x.size(0), supervisions)
+        if mask is not None:
+            mask = mask.to(x.device)
+        x = self.encoder(x, pos_emb, src_key_padding_mask=mask)  # (T, B, F)
+
+        if self.normalize_before and self.is_espnet_structure:
+            x = self.after_norm(x)
+
+        return x, mask
+
+
+class ConformerEncoderLayer(nn.Module):
+    """
+    ConformerEncoderLayer is made up of self-attn, feedforward and convolution networks.
+    See: "Conformer: Convolution-augmented Transformer for Speech Recognition"
+
+    Args:
+        d_model: the number of expected features in the input (required).
+        nhead: the number of heads in the multiheadattention models (required).
+        dim_feedforward: the dimension of the feedforward network model (default=2048).
+        dropout: the dropout value (default=0.1).
+        cnn_module_kernel (int): Kernel size of convolution module.
+        normalize_before: whether to use layer_norm before the first block.
+
+    Examples::
+        >>> encoder_layer = ConformerEncoderLayer(d_model=512, nhead=8)
+        >>> src = torch.rand(10, 32, 512)
+        >>> pos_emb = torch.rand(32, 19, 512)
+        >>> out = encoder_layer(src, pos_emb)
+    """
+
+    def __init__(
+        self,
+        d_model: int,
+        nhead: int,
+        dim_feedforward: int = 2048,
+        dropout: float = 0.1,
+        cnn_module_kernel: int = 31,
+        normalize_before: bool = True,
+        is_espnet_structure: bool = False,
+    ) -> None:
+        super(ConformerEncoderLayer, self).__init__()
+        self.self_attn = RelPositionMultiheadAttention(
+            d_model, nhead, dropout=0.0, is_espnet_structure=is_espnet_structure
+        )
+
+        self.feed_forward = nn.Sequential(
+            nn.Linear(d_model, dim_feedforward),
+            Swish(),
+            nn.Dropout(dropout),
+            nn.Linear(dim_feedforward, d_model),
+        )
+
+        self.feed_forward_macaron = nn.Sequential(
+            nn.Linear(d_model, dim_feedforward),
+            Swish(),
+            nn.Dropout(dropout),
+            nn.Linear(dim_feedforward, d_model),
+        )
+
+        self.conv_module = ConvolutionModule(d_model, cnn_module_kernel)
+
+        self.norm_ff_macaron = nn.LayerNorm(
+            d_model
+        )  # for the macaron style FNN module
+        self.norm_ff = nn.LayerNorm(d_model)  # for the FNN module
+        self.norm_mha = nn.LayerNorm(d_model)  # for the MHA module
+
+        self.ff_scale = 0.5
+
+        self.norm_conv = nn.LayerNorm(d_model)  # for the CNN module
+        self.norm_final = nn.LayerNorm(
+            d_model
+        )  # for the final output of the block
+
+        self.dropout = nn.Dropout(dropout)
+
+        self.normalize_before = normalize_before
+
+    def forward(
+        self,
+        src: Tensor,
+        pos_emb: Tensor,
+        src_mask: Optional[Tensor] = None,
+        src_key_padding_mask: Optional[Tensor] = None,
+    ) -> Tensor:
+        """
+        Pass the input through the encoder layer.
+
+        Args:
+            src: the sequence to the encoder layer (required).
+            pos_emb: Positional embedding tensor (required).
+            src_mask: the mask for the src sequence (optional).
+            src_key_padding_mask: the mask for the src keys per batch (optional).
+
+        Shape:
+            src: (S, N, E).
+            pos_emb: (N, 2*S-1, E)
+            src_mask: (S, S).
+            src_key_padding_mask: (N, S).
+            S is the source sequence length, N is the batch size, E is the feature number
+        """
+
+        # macaron style feed forward module
+        residual = src
+        if self.normalize_before:
+            src = self.norm_ff_macaron(src)
+        src = residual + self.ff_scale * self.dropout(
+            self.feed_forward_macaron(src)
+        )
+        if not self.normalize_before:
+            src = self.norm_ff_macaron(src)
+
+        # multi-headed self-attention module
+        residual = src
+        if self.normalize_before:
+            src = self.norm_mha(src)
+        src_att = self.self_attn(
+            src,
+            src,
+            src,
+            pos_emb=pos_emb,
+            attn_mask=src_mask,
+            key_padding_mask=src_key_padding_mask,
+        )[0]
+        src = residual + self.dropout(src_att)
+        if not self.normalize_before:
+            src = self.norm_mha(src)
+
+        # convolution module
+        residual = src
+        if self.normalize_before:
+            src = self.norm_conv(src)
+        src = residual + self.dropout(self.conv_module(src))
+        if not self.normalize_before:
+            src = self.norm_conv(src)
+
+        # feed forward module
+        residual = src
+        if self.normalize_before:
+            src = self.norm_ff(src)
+        src = residual + self.ff_scale * self.dropout(self.feed_forward(src))
+        if not self.normalize_before:
+            src = self.norm_ff(src)
+
+        if self.normalize_before:
+            src = self.norm_final(src)
+
+        return src
+
+
+class ConformerEncoder(nn.TransformerEncoder):
+    r"""ConformerEncoder is a stack of N encoder layers
+
+    Args:
+        encoder_layer: an instance of the ConformerEncoderLayer() class (required).
+        num_layers: the number of sub-encoder-layers in the encoder (required).
+        norm: the layer normalization component (optional).
+
+    Examples::
+        >>> encoder_layer = ConformerEncoderLayer(d_model=512, nhead=8)
+        >>> conformer_encoder = ConformerEncoder(encoder_layer, num_layers=6)
+        >>> src = torch.rand(10, 32, 512)
+        >>> pos_emb = torch.rand(32, 19, 512)
+        >>> out = conformer_encoder(src, pos_emb)
+    """
+
+    def __init__(
+        self, encoder_layer: nn.Module, num_layers: int, norm: nn.Module = None
+    ) -> None:
+        super(ConformerEncoder, self).__init__(
+            encoder_layer=encoder_layer, num_layers=num_layers, norm=norm
+        )
+
+    def forward(
+        self,
+        src: Tensor,
+        pos_emb: Tensor,
+        mask: Optional[Tensor] = None,
+        src_key_padding_mask: Optional[Tensor] = None,
+    ) -> Tensor:
+        r"""Pass the input through the encoder layers in turn.
+
+        Args:
+            src: the sequence to the encoder (required).
+            pos_emb: Positional embedding tensor (required).
+            mask: the mask for the src sequence (optional).
+            src_key_padding_mask: the mask for the src keys per batch (optional).
+
+        Shape:
+            src: (S, N, E).
+            pos_emb: (N, 2*S-1, E)
+            mask: (S, S).
+            src_key_padding_mask: (N, S).
+            S is the source sequence length, T is the target sequence length, N is the batch size, E is the feature number
+
+        """
+        output = src
+
+        for mod in self.layers:
+            output = mod(
+                output,
+                pos_emb,
+                src_mask=mask,
+                src_key_padding_mask=src_key_padding_mask,
+            )
+
+        if self.norm is not None:
+            output = self.norm(output)
+
+        return output
+
+
+class RelPositionalEncoding(torch.nn.Module):
+    """Relative positional encoding module.
+
+    See : Appendix B in "Transformer-XL: Attentive Language Models Beyond a Fixed-Length Context"
+    Modified from https://github.com/espnet/espnet/blob/master/espnet/nets/pytorch_backend/transformer/embedding.py
+
+    Args:
+        d_model: Embedding dimension.
+        dropout_rate: Dropout rate.
+        max_len: Maximum input length.
+
+    """
+
+    def __init__(
+        self, d_model: int, dropout_rate: float, max_len: int = 5000
+    ) -> None:
+        """Construct an PositionalEncoding object."""
+        super(RelPositionalEncoding, self).__init__()
+        self.d_model = d_model
+        self.xscale = math.sqrt(self.d_model)
+        self.dropout = torch.nn.Dropout(p=dropout_rate)
+        self.pe = None
+        self.extend_pe(torch.tensor(0.0).expand(1, max_len))
+
+    def extend_pe(self, x: Tensor) -> None:
+        """Reset the positional encodings."""
+        if self.pe is not None:
+            # self.pe contains both positive and negative parts
+            # the length of self.pe is 2 * input_len - 1
+            if self.pe.size(1) >= x.size(1) * 2 - 1:
+                # Note: TorchScript doesn't implement operator== for torch.Device
+                if self.pe.dtype != x.dtype or str(self.pe.device) != str(
+                    x.device
+                ):
+                    self.pe = self.pe.to(dtype=x.dtype, device=x.device)
+                return
+        # Suppose `i` means to the position of query vecotr and `j` means the
+        # position of key vector. We use position relative positions when keys
+        # are to the left (i>j) and negative relative positions otherwise (i<j).
+        pe_positive = torch.zeros(x.size(1), self.d_model)
+        pe_negative = torch.zeros(x.size(1), self.d_model)
+        position = torch.arange(0, x.size(1), dtype=torch.float32).unsqueeze(1)
+        div_term = torch.exp(
+            torch.arange(0, self.d_model, 2, dtype=torch.float32)
+            * -(math.log(10000.0) / self.d_model)
+        )
+        pe_positive[:, 0::2] = torch.sin(position * div_term)
+        pe_positive[:, 1::2] = torch.cos(position * div_term)
+        pe_negative[:, 0::2] = torch.sin(-1 * position * div_term)
+        pe_negative[:, 1::2] = torch.cos(-1 * position * div_term)
+
+        # Reserve the order of positive indices and concat both positive and
+        # negative indices. This is used to support the shifting trick
+        # as in "Transformer-XL: Attentive Language Models Beyond a Fixed-Length Context"
+        pe_positive = torch.flip(pe_positive, [0]).unsqueeze(0)
+        pe_negative = pe_negative[1:].unsqueeze(0)
+        pe = torch.cat([pe_positive, pe_negative], dim=1)
+        self.pe = pe.to(device=x.device, dtype=x.dtype)
+
+    def forward(self, x: torch.Tensor) -> Tuple[Tensor, Tensor]:
+        """Add positional encoding.
+
+        Args:
+            x (torch.Tensor): Input tensor (batch, time, `*`).
+
+        Returns:
+            torch.Tensor: Encoded tensor (batch, time, `*`).
+            torch.Tensor: Encoded tensor (batch, 2*time-1, `*`).
+
+        """
+        self.extend_pe(x)
+        x = x * self.xscale
+        pos_emb = self.pe[
+            :,
+            self.pe.size(1) // 2
+            - x.size(1)
+            + 1 : self.pe.size(1) // 2  # noqa E203
+            + x.size(1),
+        ]
+        return self.dropout(x), self.dropout(pos_emb)
+
+
+class RelPositionMultiheadAttention(nn.Module):
+    r"""Multi-Head Attention layer with relative position encoding
+
+    See reference: "Transformer-XL: Attentive Language Models Beyond a Fixed-Length Context"
+
+    Args:
+        embed_dim: total dimension of the model.
+        num_heads: parallel attention heads.
+        dropout: a Dropout layer on attn_output_weights. Default: 0.0.
+
+    Examples::
+
+        >>> rel_pos_multihead_attn = RelPositionMultiheadAttention(embed_dim, num_heads)
+        >>> attn_output, attn_output_weights = multihead_attn(query, key, value, pos_emb)
+    """
+
+    def __init__(
+        self,
+        embed_dim: int,
+        num_heads: int,
+        dropout: float = 0.0,
+        is_espnet_structure: bool = False,
+    ) -> None:
+        super(RelPositionMultiheadAttention, self).__init__()
+        self.embed_dim = embed_dim
+        self.num_heads = num_heads
+        self.dropout = dropout
+        self.head_dim = embed_dim // num_heads
+        assert (
+            self.head_dim * num_heads == self.embed_dim
+        ), "embed_dim must be divisible by num_heads"
+
+        self.in_proj = nn.Linear(embed_dim, 3 * embed_dim, bias=True)
+        self.out_proj = nn.Linear(embed_dim, embed_dim, bias=True)
+
+        # linear transformation for positional encoding.
+        self.linear_pos = nn.Linear(embed_dim, embed_dim, bias=False)
+        # these two learnable bias are used in matrix c and matrix d
+        # as described in "Transformer-XL: Attentive Language Models Beyond a Fixed-Length Context" Section 3.3
+        self.pos_bias_u = nn.Parameter(torch.Tensor(num_heads, self.head_dim))
+        self.pos_bias_v = nn.Parameter(torch.Tensor(num_heads, self.head_dim))
+
+        self._reset_parameters()
+
+        self.is_espnet_structure = is_espnet_structure
+
+    def _reset_parameters(self) -> None:
+        nn.init.xavier_uniform_(self.in_proj.weight)
+        nn.init.constant_(self.in_proj.bias, 0.0)
+        nn.init.constant_(self.out_proj.bias, 0.0)
+
+        nn.init.xavier_uniform_(self.pos_bias_u)
+        nn.init.xavier_uniform_(self.pos_bias_v)
+
+    def forward(
+        self,
+        query: Tensor,
+        key: Tensor,
+        value: Tensor,
+        pos_emb: Tensor,
+        key_padding_mask: Optional[Tensor] = None,
+        need_weights: bool = True,
+        attn_mask: Optional[Tensor] = None,
+    ) -> Tuple[Tensor, Optional[Tensor]]:
+        r"""
+        Args:
+            query, key, value: map a query and a set of key-value pairs to an output.
+            pos_emb: Positional embedding tensor
+            key_padding_mask: if provided, specified padding elements in the key will
+                be ignored by the attention. When given a binary mask and a value is True,
+                the corresponding value on the attention layer will be ignored. When given
+                a byte mask and a value is non-zero, the corresponding value on the attention
+                layer will be ignored
+            need_weights: output attn_output_weights.
+            attn_mask: 2D or 3D mask that prevents attention to certain positions. A 2D mask will be broadcasted for all
+                the batches while a 3D mask allows to specify a different mask for the entries of each batch.
+
+        Shape:
+            - Inputs:
+            - query: :math:`(L, N, E)` where L is the target sequence length, N is the batch size, E is
+            the embedding dimension.
+            - key: :math:`(S, N, E)`, where S is the source sequence length, N is the batch size, E is
+            the embedding dimension.
+            - value: :math:`(S, N, E)` where S is the source sequence length, N is the batch size, E is
+            the embedding dimension.
+            - pos_emb: :math:`(N, 2*L-1, E)` where L is the target sequence length, N is the batch size, E is
+            the embedding dimension.
+            - key_padding_mask: :math:`(N, S)` where N is the batch size, S is the source sequence length.
+            If a ByteTensor is provided, the non-zero positions will be ignored while the position
+            with the zero positions will be unchanged. If a BoolTensor is provided, the positions with the
+            value of ``True`` will be ignored while the position with the value of ``False`` will be unchanged.
+            - attn_mask: 2D mask :math:`(L, S)` where L is the target sequence length, S is the source sequence length.
+            3D mask :math:`(N*num_heads, L, S)` where N is the batch size, L is the target sequence length,
+            S is the source sequence length. attn_mask ensure that position i is allowed to attend the unmasked
+            positions. If a ByteTensor is provided, the non-zero positions are not allowed to attend
+            while the zero positions will be unchanged. If a BoolTensor is provided, positions with ``True``
+            is not allowed to attend while ``False`` values will be unchanged. If a FloatTensor
+            is provided, it will be added to the attention weight.
+
+            - Outputs:
+            - attn_output: :math:`(L, N, E)` where L is the target sequence length, N is the batch size,
+            E is the embedding dimension.
+            - attn_output_weights: :math:`(N, L, S)` where N is the batch size,
+            L is the target sequence length, S is the source sequence length.
+        """
+        return self.multi_head_attention_forward(
+            query,
+            key,
+            value,
+            pos_emb,
+            self.embed_dim,
+            self.num_heads,
+            self.in_proj.weight,
+            self.in_proj.bias,
+            self.dropout,
+            self.out_proj.weight,
+            self.out_proj.bias,
+            training=self.training,
+            key_padding_mask=key_padding_mask,
+            need_weights=need_weights,
+            attn_mask=attn_mask,
+        )
+
+    def rel_shift(self, x: Tensor) -> Tensor:
+        """Compute relative positional encoding.
+
+        Args:
+            x: Input tensor (batch, head, time1, 2*time1-1).
+                time1 means the length of query vector.
+
+        Returns:
+            Tensor: tensor of shape (batch, head, time1, time2)
+          (note: time2 has the same value as time1, but it is for
+          the key, while time1 is for the query).
+        """
+        (batch_size, num_heads, time1, n) = x.shape
+        assert n == 2 * time1 - 1
+        # Note: TorchScript requires explicit arg for stride()
+        batch_stride = x.stride(0)
+        head_stride = x.stride(1)
+        time1_stride = x.stride(2)
+        n_stride = x.stride(3)
+        return x.as_strided(
+            (batch_size, num_heads, time1, time1),
+            (batch_stride, head_stride, time1_stride - n_stride, n_stride),
+            storage_offset=n_stride * (time1 - 1),
+        )
+
+    def multi_head_attention_forward(
+        self,
+        query: Tensor,
+        key: Tensor,
+        value: Tensor,
+        pos_emb: Tensor,
+        embed_dim_to_check: int,
+        num_heads: int,
+        in_proj_weight: Tensor,
+        in_proj_bias: Tensor,
+        dropout_p: float,
+        out_proj_weight: Tensor,
+        out_proj_bias: Tensor,
+        training: bool = True,
+        key_padding_mask: Optional[Tensor] = None,
+        need_weights: bool = True,
+        attn_mask: Optional[Tensor] = None,
+    ) -> Tuple[Tensor, Optional[Tensor]]:
+        r"""
+        Args:
+            query, key, value: map a query and a set of key-value pairs to an output.
+            pos_emb: Positional embedding tensor
+            embed_dim_to_check: total dimension of the model.
+            num_heads: parallel attention heads.
+            in_proj_weight, in_proj_bias: input projection weight and bias.
+            dropout_p: probability of an element to be zeroed.
+            out_proj_weight, out_proj_bias: the output projection weight and bias.
+            training: apply dropout if is ``True``.
+            key_padding_mask: if provided, specified padding elements in the key will
+                be ignored by the attention. This is an binary mask. When the value is True,
+                the corresponding value on the attention layer will be filled with -inf.
+            need_weights: output attn_output_weights.
+            attn_mask: 2D or 3D mask that prevents attention to certain positions. A 2D mask will be broadcasted for all
+                the batches while a 3D mask allows to specify a different mask for the entries of each batch.
+
+        Shape:
+            Inputs:
+            - query: :math:`(L, N, E)` where L is the target sequence length, N is the batch size, E is
+            the embedding dimension.
+            - key: :math:`(S, N, E)`, where S is the source sequence length, N is the batch size, E is
+            the embedding dimension.
+            - value: :math:`(S, N, E)` where S is the source sequence length, N is the batch size, E is
+            the embedding dimension.
+            - pos_emb: :math:`(N, 2*L-1, E)` or :math:`(1, 2*L-1, E)` where L is the target sequence
+            length, N is the batch size, E is the embedding dimension.
+            - key_padding_mask: :math:`(N, S)` where N is the batch size, S is the source sequence length.
+            If a ByteTensor is provided, the non-zero positions will be ignored while the zero positions
+            will be unchanged. If a BoolTensor is provided, the positions with the
+            value of ``True`` will be ignored while the position with the value of ``False`` will be unchanged.
+            - attn_mask: 2D mask :math:`(L, S)` where L is the target sequence length, S is the source sequence length.
+            3D mask :math:`(N*num_heads, L, S)` where N is the batch size, L is the target sequence length,
+            S is the source sequence length. attn_mask ensures that position i is allowed to attend the unmasked
+            positions. If a ByteTensor is provided, the non-zero positions are not allowed to attend
+            while the zero positions will be unchanged. If a BoolTensor is provided, positions with ``True``
+            are not allowed to attend while ``False`` values will be unchanged. If a FloatTensor
+            is provided, it will be added to the attention weight.
+
+            Outputs:
+            - attn_output: :math:`(L, N, E)` where L is the target sequence length, N is the batch size,
+            E is the embedding dimension.
+            - attn_output_weights: :math:`(N, L, S)` where N is the batch size,
+            L is the target sequence length, S is the source sequence length.
+        """
+
+        tgt_len, bsz, embed_dim = query.size()
+        assert embed_dim == embed_dim_to_check
+        assert key.size(0) == value.size(0) and key.size(1) == value.size(1)
+
+        head_dim = embed_dim // num_heads
+        assert (
+            head_dim * num_heads == embed_dim
+        ), "embed_dim must be divisible by num_heads"
+        scaling = float(head_dim) ** -0.5
+
+        if torch.equal(query, key) and torch.equal(key, value):
+            # self-attention
+            q, k, v = nn.functional.linear(
+                query, in_proj_weight, in_proj_bias
+            ).chunk(3, dim=-1)
+
+        elif torch.equal(key, value):
+            # encoder-decoder attention
+            # This is inline in_proj function with in_proj_weight and in_proj_bias
+            _b = in_proj_bias
+            _start = 0
+            _end = embed_dim
+            _w = in_proj_weight[_start:_end, :]
+            if _b is not None:
+                _b = _b[_start:_end]
+            q = nn.functional.linear(query, _w, _b)
+            # This is inline in_proj function with in_proj_weight and in_proj_bias
+            _b = in_proj_bias
+            _start = embed_dim
+            _end = None
+            _w = in_proj_weight[_start:, :]
+            if _b is not None:
+                _b = _b[_start:]
+            k, v = nn.functional.linear(key, _w, _b).chunk(2, dim=-1)
+
+        else:
+            # This is inline in_proj function with in_proj_weight and in_proj_bias
+            _b = in_proj_bias
+            _start = 0
+            _end = embed_dim
+            _w = in_proj_weight[_start:_end, :]
+            if _b is not None:
+                _b = _b[_start:_end]
+            q = nn.functional.linear(query, _w, _b)
+
+            # This is inline in_proj function with in_proj_weight and in_proj_bias
+            _b = in_proj_bias
+            _start = embed_dim
+            _end = embed_dim * 2
+            _w = in_proj_weight[_start:_end, :]
+            if _b is not None:
+                _b = _b[_start:_end]
+            k = nn.functional.linear(key, _w, _b)
+
+            # This is inline in_proj function with in_proj_weight and in_proj_bias
+            _b = in_proj_bias
+            _start = embed_dim * 2
+            _end = None
+            _w = in_proj_weight[_start:, :]
+            if _b is not None:
+                _b = _b[_start:]
+            v = nn.functional.linear(value, _w, _b)
+
+        if not self.is_espnet_structure:
+            q = q * scaling
+
+        if attn_mask is not None:
+            assert (
+                attn_mask.dtype == torch.float32
+                or attn_mask.dtype == torch.float64
+                or attn_mask.dtype == torch.float16
+                or attn_mask.dtype == torch.uint8
+                or attn_mask.dtype == torch.bool
+            ), "Only float, byte, and bool types are supported for attn_mask, not {}".format(
+                attn_mask.dtype
+            )
+            if attn_mask.dtype == torch.uint8:
+                warnings.warn(
+                    "Byte tensor for attn_mask is deprecated. Use bool tensor instead."
+                )
+                attn_mask = attn_mask.to(torch.bool)
+
+            if attn_mask.dim() == 2:
+                attn_mask = attn_mask.unsqueeze(0)
+                if list(attn_mask.size()) != [1, query.size(0), key.size(0)]:
+                    raise RuntimeError(
+                        "The size of the 2D attn_mask is not correct."
+                    )
+            elif attn_mask.dim() == 3:
+                if list(attn_mask.size()) != [
+                    bsz * num_heads,
+                    query.size(0),
+                    key.size(0),
+                ]:
+                    raise RuntimeError(
+                        "The size of the 3D attn_mask is not correct."
+                    )
+            else:
+                raise RuntimeError(
+                    "attn_mask's dimension {} is not supported".format(
+                        attn_mask.dim()
+                    )
+                )
+            # attn_mask's dim is 3 now.
+
+        # convert ByteTensor key_padding_mask to bool
+        if (
+            key_padding_mask is not None
+            and key_padding_mask.dtype == torch.uint8
+        ):
+            warnings.warn(
+                "Byte tensor for key_padding_mask is deprecated. Use bool tensor instead."
+            )
+            key_padding_mask = key_padding_mask.to(torch.bool)
+
+        q = q.contiguous().view(tgt_len, bsz, num_heads, head_dim)
+        k = k.contiguous().view(-1, bsz, num_heads, head_dim)
+        v = v.contiguous().view(-1, bsz * num_heads, head_dim).transpose(0, 1)
+
+        src_len = k.size(0)
+
+        if key_padding_mask is not None:
+            assert key_padding_mask.size(0) == bsz, "{} == {}".format(
+                key_padding_mask.size(0), bsz
+            )
+            assert key_padding_mask.size(1) == src_len, "{} == {}".format(
+                key_padding_mask.size(1), src_len
+            )
+
+        q = q.transpose(0, 1)  # (batch, time1, head, d_k)
+
+        pos_emb_bsz = pos_emb.size(0)
+        assert pos_emb_bsz in (1, bsz)  # actually it is 1
+        p = self.linear_pos(pos_emb).view(pos_emb_bsz, -1, num_heads, head_dim)
+        p = p.transpose(1, 2)  # (batch, head, 2*time1-1, d_k)
+
+        q_with_bias_u = (q + self.pos_bias_u).transpose(
+            1, 2
+        )  # (batch, head, time1, d_k)
+
+        q_with_bias_v = (q + self.pos_bias_v).transpose(
+            1, 2
+        )  # (batch, head, time1, d_k)
+
+        # compute attention score
+        # first compute matrix a and matrix c
+        # as described in "Transformer-XL: Attentive Language Models Beyond a Fixed-Length Context" Section 3.3
+        k = k.permute(1, 2, 3, 0)  # (batch, head, d_k, time2)
+        matrix_ac = torch.matmul(
+            q_with_bias_u, k
+        )  # (batch, head, time1, time2)
+
+        # compute matrix b and matrix d
+        matrix_bd = torch.matmul(
+            q_with_bias_v, p.transpose(-2, -1)
+        )  # (batch, head, time1, 2*time1-1)
+        matrix_bd = self.rel_shift(matrix_bd)
+
+        if not self.is_espnet_structure:
+            attn_output_weights = (
+                matrix_ac + matrix_bd
+            )  # (batch, head, time1, time2)
+        else:
+            attn_output_weights = (
+                matrix_ac + matrix_bd
+            ) * scaling  # (batch, head, time1, time2)
+
+        attn_output_weights = attn_output_weights.view(
+            bsz * num_heads, tgt_len, -1
+        )
+
+        assert list(attn_output_weights.size()) == [
+            bsz * num_heads,
+            tgt_len,
+            src_len,
+        ]
+
+        if attn_mask is not None:
+            if attn_mask.dtype == torch.bool:
+                attn_output_weights.masked_fill_(attn_mask, float("-inf"))
+            else:
+                attn_output_weights += attn_mask
+
+        if key_padding_mask is not None:
+            attn_output_weights = attn_output_weights.view(
+                bsz, num_heads, tgt_len, src_len
+            )
+            attn_output_weights = attn_output_weights.masked_fill(
+                key_padding_mask.unsqueeze(1).unsqueeze(2),
+                float("-inf"),
+            )
+            attn_output_weights = attn_output_weights.view(
+                bsz * num_heads, tgt_len, src_len
+            )
+
+        attn_output_weights = nn.functional.softmax(attn_output_weights, dim=-1)
+        attn_output_weights = nn.functional.dropout(
+            attn_output_weights, p=dropout_p, training=training
+        )
+
+        attn_output = torch.bmm(attn_output_weights, v)
+        assert list(attn_output.size()) == [bsz * num_heads, tgt_len, head_dim]
+        attn_output = (
+            attn_output.transpose(0, 1)
+            .contiguous()
+            .view(tgt_len, bsz, embed_dim)
+        )
+        attn_output = nn.functional.linear(
+            attn_output, out_proj_weight, out_proj_bias
+        )
+
+        if need_weights:
+            # average attention weights over heads
+            attn_output_weights = attn_output_weights.view(
+                bsz, num_heads, tgt_len, src_len
+            )
+            return attn_output, attn_output_weights.sum(dim=1) / num_heads
+        else:
+            return attn_output, None
+
+
+class ConvolutionModule(nn.Module):
+    """ConvolutionModule in Conformer model.
+    Modified from https://github.com/espnet/espnet/blob/master/espnet/nets/pytorch_backend/conformer/convolution.py
+
+    Args:
+        channels (int): The number of channels of conv layers.
+        kernel_size (int): Kernerl size of conv layers.
+        bias (bool): Whether to use bias in conv layers (default=True).
+
+    """
+
+    def __init__(
+        self, channels: int, kernel_size: int, bias: bool = True
+    ) -> None:
+        """Construct an ConvolutionModule object."""
+        super(ConvolutionModule, self).__init__()
+        # kernerl_size should be a odd number for 'SAME' padding
+        assert (kernel_size - 1) % 2 == 0
+
+        self.pointwise_conv1 = nn.Conv1d(
+            channels,
+            2 * channels,
+            kernel_size=1,
+            stride=1,
+            padding=0,
+            bias=bias,
+        )
+        self.depthwise_conv = nn.Conv1d(
+            channels,
+            channels,
+            kernel_size,
+            stride=1,
+            padding=(kernel_size - 1) // 2,
+            groups=channels,
+            bias=bias,
+        )
+        self.norm = nn.BatchNorm1d(channels)
+        self.pointwise_conv2 = nn.Conv1d(
+            channels,
+            channels,
+            kernel_size=1,
+            stride=1,
+            padding=0,
+            bias=bias,
+        )
+        self.activation = Swish()
+
+    def forward(self, x: Tensor) -> Tensor:
+        """Compute convolution module.
+
+        Args:
+            x: Input tensor (#time, batch, channels).
+
+        Returns:
+            Tensor: Output tensor (#time, batch, channels).
+
+        """
+        # exchange the temporal dimension and the feature dimension
+        x = x.permute(1, 2, 0)  # (#batch, channels, time).
+
+        # GLU mechanism
+        x = self.pointwise_conv1(x)  # (batch, 2*channels, time)
+        x = nn.functional.glu(x, dim=1)  # (batch, channels, time)
+
+        # 1D Depthwise Conv
+        x = self.depthwise_conv(x)
+        x = self.activation(self.norm(x))
+
+        x = self.pointwise_conv2(x)  # (batch, channel, time)
+
+        return x.permute(2, 0, 1)
+
+
+class Swish(torch.nn.Module):
+    """Construct an Swish object."""
+
+    def forward(self, x: Tensor) -> Tensor:
+        """Return Swich activation function."""
+        return x * torch.sigmoid(x)
+
+
+def identity(x):
+    return x

egs/librispeech/ASR/conformer_mmi_phone/decode.py

@@ -0,0 +1,548 @@
+#!/usr/bin/env python3
+
+# Copyright 2021 Xiaomi Corporation (Author: Liyong Guo, Fangjun Kuang)
+
+# (still working in progress)
+
+import argparse
+import logging
+from collections import defaultdict
+from pathlib import Path
+from typing import Dict, List, Optional, Tuple
+
+import k2
+import torch
+import torch.nn as nn
+from asr_datamodule import LibriSpeechAsrDataModule
+from conformer import Conformer
+
+from icefall.bpe_graph_compiler import BpeCtcTrainingGraphCompiler
+from icefall.checkpoint import average_checkpoints, load_checkpoint
+from icefall.decode import (
+    get_lattice,
+    nbest_decoding,
+    nbest_oracle,
+    one_best_decoding,
+    rescore_with_attention_decoder,
+    rescore_with_n_best_list,
+    rescore_with_whole_lattice,
+)
+from icefall.lexicon import Lexicon
+from icefall.utils import (
+    AttributeDict,
+    get_texts,
+    setup_logger,
+    store_transcripts,
+    write_error_stats,
+)
+
+
+def get_parser():
+    parser = argparse.ArgumentParser(
+        formatter_class=argparse.ArgumentDefaultsHelpFormatter
+    )
+
+    parser.add_argument(
+        "--epoch",
+        type=int,
+        default=9,
+        help="It specifies the checkpoint to use for decoding."
+        "Note: Epoch counts from 0.",
+    )
+    parser.add_argument(
+        "--avg",
+        type=int,
+        default=1,
+        help="Number of checkpoints to average. Automatically select "
+        "consecutive checkpoints before the checkpoint specified by "
+        "'--epoch'. ",
+    )
+
+    parser.add_argument(
+        "--lattice-score-scale",
+        type=float,
+        default=1.0,
+        help="The scale to be applied to `lattice.scores`."
+        "It's needed if you use any kinds of n-best based rescoring. "
+        "Currently, it is used when the decoding method is: nbest, "
+        "nbest-rescoring, attention-decoder, and nbest-oracle. "
+        "A smaller value results in more unique paths.",
+    )
+
+    return parser
+
+
+def get_params() -> AttributeDict:
+    params = AttributeDict(
+        {
+            "exp_dir": Path("conformer_mmi_phone/exp"),
+            "lang_dir": Path("data/lang_phone"),
+            "lm_dir": Path("data/lm"),
+            "feature_dim": 80,
+            "nhead": 8,
+            "attention_dim": 512,
+            "subsampling_factor": 4,
+            "num_decoder_layers": 6,
+            "vgg_frontend": False,
+            "is_espnet_structure": True,
+            "mmi_loss": False,
+            "use_feat_batchnorm": True,
+            "search_beam": 20,
+            "output_beam": 8,
+            "min_active_states": 30,
+            "max_active_states": 10000,
+            "use_double_scores": True,
+            # Possible values for method:
+            #  - 1best
+            #  - nbest
+            #  - nbest-rescoring
+            #  - whole-lattice-rescoring
+            #  - attention-decoder
+            #  - nbest-oracle
+            #  "method": "nbest",
+            #  "method": "nbest-rescoring",
+            #  "method": "whole-lattice-rescoring",
+            "method": "attention-decoder",
+            #  "method": "nbest-oracle",
+            # num_paths is used when method is "nbest", "nbest-rescoring",
+            # attention-decoder, and nbest-oracle
+            "num_paths": 100,
+        }
+    )
+    return params
+
+
+def decode_one_batch(
+    params: AttributeDict,
+    model: nn.Module,
+    HLG: k2.Fsa,
+    batch: dict,
+    lexicon: Lexicon,
+    sos_id: int,
+    eos_id: int,
+    G: Optional[k2.Fsa] = None,
+) -> Dict[str, List[List[int]]]:
+    """Decode one batch and return the result in a dict. The dict has the
+    following format:
+
+        - key: It indicates the setting used for decoding. For example,
+               if no rescoring is used, the key is the string `no_rescore`.
+               If LM rescoring is used, the key is the string `lm_scale_xxx`,
+               where `xxx` is the value of `lm_scale`. An example key is
+               `lm_scale_0.7`
+        - value: It contains the decoding result. `len(value)` equals to
+                 batch size. `value[i]` is the decoding result for the i-th
+                 utterance in the given batch.
+    Args:
+      params:
+        It's the return value of :func:`get_params`.
+
+        - params.method is "1best", it uses 1best decoding without LM rescoring.
+        - params.method is "nbest", it uses nbest decoding without LM rescoring.
+        - params.method is "nbest-rescoring", it uses nbest LM rescoring.
+        - params.method is "whole-lattice-rescoring", it uses whole lattice LM
+          rescoring.
+
+      model:
+        The neural model.
+      HLG:
+        The decoding graph.
+      batch:
+        It is the return value from iterating
+        `lhotse.dataset.K2SpeechRecognitionDataset`. See its documentation
+        for the format of the `batch`.
+      lexicon:
+        It contains word symbol table.
+      sos_id:
+        The token ID of the SOS.
+      eos_id:
+        The token ID of the EOS.
+      G:
+        An LM. It is not None when params.method is "nbest-rescoring"
+        or "whole-lattice-rescoring". In general, the G in HLG
+        is a 3-gram LM, while this G is a 4-gram LM.
+    Returns:
+      Return the decoding result. See above description for the format of
+      the returned dict.
+    """
+    device = HLG.device
+    feature = batch["inputs"]
+    assert feature.ndim == 3
+    feature = feature.to(device)
+    # at entry, feature is [N, T, C]
+
+    supervisions = batch["supervisions"]
+
+    nnet_output, memory, memory_key_padding_mask = model(feature, supervisions)
+    # nnet_output is [N, T, C]
+
+    supervision_segments = torch.stack(
+        (
+            supervisions["sequence_idx"],
+            supervisions["start_frame"] // params.subsampling_factor,
+            supervisions["num_frames"] // params.subsampling_factor,
+        ),
+        1,
+    ).to(torch.int32)
+
+    lattice = get_lattice(
+        nnet_output=nnet_output,
+        HLG=HLG,
+        supervision_segments=supervision_segments,
+        search_beam=params.search_beam,
+        output_beam=params.output_beam,
+        min_active_states=params.min_active_states,
+        max_active_states=params.max_active_states,
+        subsampling_factor=params.subsampling_factor,
+    )
+
+    if params.method == "nbest-oracle":
+        # Note: You can also pass rescored lattices to it.
+        # We choose the HLG decoded lattice for speed reasons
+        # as HLG decoding is faster and the oracle WER
+        # is slightly worse than that of rescored lattices.
+        return nbest_oracle(
+            lattice=lattice,
+            num_paths=params.num_paths,
+            ref_texts=supervisions["text"],
+            lexicon=lexicon,
+            scale=params.lattice_score_scale,
+        )
+
+    if params.method in ["1best", "nbest"]:
+        if params.method == "1best":
+            best_path = one_best_decoding(
+                lattice=lattice, use_double_scores=params.use_double_scores
+            )
+            key = "no_rescore"
+        else:
+            best_path = nbest_decoding(
+                lattice=lattice,
+                num_paths=params.num_paths,
+                use_double_scores=params.use_double_scores,
+                scale=params.lattice_score_scale,
+            )
+            key = f"no_rescore-scale-{params.lattice_score_scale}-{params.num_paths}"  # noqa
+
+        hyps = get_texts(best_path)
+        hyps = [[lexicon.word_table[i] for i in ids] for ids in hyps]
+        return {key: hyps}
+
+    assert params.method in [
+        "nbest-rescoring",
+        "whole-lattice-rescoring",
+        "attention-decoder",
+    ]
+
+    lm_scale_list = [0.8, 0.9, 1.0, 1.1, 1.2, 1.3]
+    lm_scale_list += [1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0]
+
+    if params.method == "nbest-rescoring":
+        best_path_dict = rescore_with_n_best_list(
+            lattice=lattice,
+            G=G,
+            num_paths=params.num_paths,
+            lm_scale_list=lm_scale_list,
+            scale=params.lattice_score_scale,
+        )
+    elif params.method == "whole-lattice-rescoring":
+        best_path_dict = rescore_with_whole_lattice(
+            lattice=lattice, G_with_epsilon_loops=G, lm_scale_list=lm_scale_list
+        )
+    elif params.method == "attention-decoder":
+        # lattice uses a 3-gram Lm. We rescore it with a 4-gram LM.
+        rescored_lattice = rescore_with_whole_lattice(
+            lattice=lattice, G_with_epsilon_loops=G, lm_scale_list=None
+        )
+
+        best_path_dict = rescore_with_attention_decoder(
+            lattice=rescored_lattice,
+            num_paths=params.num_paths,
+            model=model,
+            memory=memory,
+            memory_key_padding_mask=memory_key_padding_mask,
+            sos_id=sos_id,
+            eos_id=eos_id,
+            scale=params.lattice_score_scale,
+        )
+    else:
+        assert False, f"Unsupported decoding method: {params.method}"
+
+    ans = dict()
+    for lm_scale_str, best_path in best_path_dict.items():
+        hyps = get_texts(best_path)
+        hyps = [[lexicon.word_table[i] for i in ids] for ids in hyps]
+        ans[lm_scale_str] = hyps
+    return ans
+
+
+def decode_dataset(
+    dl: torch.utils.data.DataLoader,
+    params: AttributeDict,
+    model: nn.Module,
+    HLG: k2.Fsa,
+    lexicon: Lexicon,
+    sos_id: int,
+    eos_id: int,
+    G: Optional[k2.Fsa] = None,
+) -> Dict[str, List[Tuple[List[int], List[int]]]]:
+    """Decode dataset.
+
+    Args:
+      dl:
+        PyTorch's dataloader containing the dataset to decode.
+      params:
+        It is returned by :func:`get_params`.
+      model:
+        The neural model.
+      HLG:
+        The decoding graph.
+      lexicon:
+        It contains word symbol table.
+      sos_id:
+        The token ID for SOS.
+      eos_id:
+        The token ID for EOS.
+      G:
+        An LM. It is not None when params.method is "nbest-rescoring"
+        or "whole-lattice-rescoring". In general, the G in HLG
+        is a 3-gram LM, while this G is a 4-gram LM.
+    Returns:
+      Return a dict, whose key may be "no-rescore" if no LM rescoring
+      is used, or it may be "lm_scale_0.7" if LM rescoring is used.
+      Its value is a list of tuples. Each tuple contains two elements:
+      The first is the reference transcript, and the second is the
+      predicted result.
+    """
+    results = []
+
+    num_cuts = 0
+
+    try:
+        num_batches = len(dl)
+    except TypeError:
+        num_batches = "?"
+
+    results = defaultdict(list)
+    for batch_idx, batch in enumerate(dl):
+        texts = batch["supervisions"]["text"]
+
+        hyps_dict = decode_one_batch(
+            params=params,
+            model=model,
+            HLG=HLG,
+            batch=batch,
+            lexicon=lexicon,
+            G=G,
+            sos_id=sos_id,
+            eos_id=eos_id,
+        )
+
+        for lm_scale, hyps in hyps_dict.items():
+            this_batch = []
+            assert len(hyps) == len(texts)
+            for hyp_words, ref_text in zip(hyps, texts):
+                ref_words = ref_text.split()
+                this_batch.append((ref_words, hyp_words))
+
+            results[lm_scale].extend(this_batch)
+
+        num_cuts += len(batch["supervisions"]["text"])
+
+        if batch_idx % 100 == 0:
+            batch_str = f"{batch_idx}/{num_batches}"
+
+            logging.info(
+                f"batch {batch_str}, cuts processed until now is {num_cuts}"
+            )
+    return results
+
+
+def save_results(
+    params: AttributeDict,
+    test_set_name: str,
+    results_dict: Dict[str, List[Tuple[List[int], List[int]]]],
+):
+    if params.method == "attention-decoder":
+        # Set it to False since there are too many logs.
+        enable_log = False
+    else:
+        enable_log = True
+    test_set_wers = dict()
+    for key, results in results_dict.items():
+        recog_path = params.exp_dir / f"recogs-{test_set_name}-{key}.txt"
+        store_transcripts(filename=recog_path, texts=results)
+        if enable_log:
+            logging.info(f"The transcripts are stored in {recog_path}")
+
+        # The following prints out WERs, per-word error statistics and aligned
+        # ref/hyp pairs.
+        errs_filename = params.exp_dir / f"errs-{test_set_name}-{key}.txt"
+        with open(errs_filename, "w") as f:
+            wer = write_error_stats(
+                f, f"{test_set_name}-{key}", results, enable_log=enable_log
+            )
+            test_set_wers[key] = wer
+
+        if enable_log:
+            logging.info(
+                "Wrote detailed error stats to {}".format(errs_filename)
+            )
+
+    test_set_wers = sorted(test_set_wers.items(), key=lambda x: x[1])
+    errs_info = params.exp_dir / f"wer-summary-{test_set_name}.txt"
+    with open(errs_info, "w") as f:
+        print("settings\tWER", file=f)
+        for key, val in test_set_wers:
+            print("{}\t{}".format(key, val), file=f)
+
+    s = "\nFor {}, WER of different settings are:\n".format(test_set_name)
+    note = "\tbest for {}".format(test_set_name)
+    for key, val in test_set_wers:
+        s += "{}\t{}{}\n".format(key, val, note)
+        note = ""
+    logging.info(s)
+
+
+@torch.no_grad()
+def main():
+    parser = get_parser()
+    LibriSpeechAsrDataModule.add_arguments(parser)
+    args = parser.parse_args()
+
+    params = get_params()
+    params.update(vars(args))
+
+    setup_logger(f"{params.exp_dir}/log-{params.method}/log-decode")
+    logging.info("Decoding started")
+    logging.info(params)
+
+    lexicon = Lexicon(params.lang_dir)
+    max_token_id = max(lexicon.tokens)
+    num_classes = max_token_id + 1  # +1 for the blank
+
+    device = torch.device("cpu")
+    if torch.cuda.is_available():
+        device = torch.device("cuda", 0)
+
+    logging.info(f"device: {device}")
+
+    graph_compiler = BpeCtcTrainingGraphCompiler(
+        params.lang_dir,
+        device=device,
+        sos_token="<sos/eos>",
+        eos_token="<sos/eos>",
+    )
+    sos_id = graph_compiler.sos_id
+    eos_id = graph_compiler.eos_id
+
+    HLG = k2.Fsa.from_dict(
+        torch.load(f"{params.lang_dir}/HLG.pt", map_location="cpu")
+    )
+    HLG = HLG.to(device)
+    assert HLG.requires_grad is False
+
+    if not hasattr(HLG, "lm_scores"):
+        HLG.lm_scores = HLG.scores.clone()
+
+    if params.method in (
+        "nbest-rescoring",
+        "whole-lattice-rescoring",
+        "attention-decoder",
+    ):
+        if not (params.lm_dir / "G_4_gram.pt").is_file():
+            logging.info("Loading G_4_gram.fst.txt")
+            logging.warning("It may take 8 minutes.")
+            with open(params.lm_dir / "G_4_gram.fst.txt") as f:
+                first_word_disambig_id = lexicon.word_table["#0"]
+
+                G = k2.Fsa.from_openfst(f.read(), acceptor=False)
+                # G.aux_labels is not needed in later computations, so
+                # remove it here.
+                del G.aux_labels
+                # CAUTION: The following line is crucial.
+                # Arcs entering the back-off state have label equal to #0.
+                # We have to change it to 0 here.
+                G.labels[G.labels >= first_word_disambig_id] = 0
+                G = k2.Fsa.from_fsas([G]).to(device)
+                G = k2.arc_sort(G)
+                torch.save(G.as_dict(), params.lm_dir / "G_4_gram.pt")
+        else:
+            logging.info("Loading pre-compiled G_4_gram.pt")
+            d = torch.load(params.lm_dir / "G_4_gram.pt", map_location="cpu")
+            G = k2.Fsa.from_dict(d).to(device)
+
+        if params.method in ["whole-lattice-rescoring", "attention-decoder"]:
+            # Add epsilon self-loops to G as we will compose
+            # it with the whole lattice later
+            G = k2.add_epsilon_self_loops(G)
+            G = k2.arc_sort(G)
+            G = G.to(device)
+
+        # G.lm_scores is used to replace HLG.lm_scores during
+        # LM rescoring.
+        G.lm_scores = G.scores.clone()
+    else:
+        G = None
+
+    model = Conformer(
+        num_features=params.feature_dim,
+        nhead=params.nhead,
+        d_model=params.attention_dim,
+        num_classes=num_classes,
+        subsampling_factor=params.subsampling_factor,
+        num_decoder_layers=params.num_decoder_layers,
+        vgg_frontend=params.vgg_frontend,
+        is_espnet_structure=params.is_espnet_structure,
+        mmi_loss=params.mmi_loss,
+        use_feat_batchnorm=params.use_feat_batchnorm,
+    )
+
+    if params.avg == 1:
+        load_checkpoint(f"{params.exp_dir}/epoch-{params.epoch}.pt", model)
+    else:
+        start = params.epoch - params.avg + 1
+        filenames = []
+        for i in range(start, params.epoch + 1):
+            if start >= 0:
+                filenames.append(f"{params.exp_dir}/epoch-{i}.pt")
+        logging.info(f"averaging {filenames}")
+        model.load_state_dict(average_checkpoints(filenames))
+
+    model.to(device)
+    model.eval()
+    num_param = sum([p.numel() for p in model.parameters()])
+    logging.info(f"Number of model parameters: {num_param}")
+
+    librispeech = LibriSpeechAsrDataModule(args)
+    # CAUTION: `test_sets` is for displaying only.
+    # If you want to skip test-clean, you have to skip
+    # it inside the for loop. That is, use
+    #
+    #   if test_set == 'test-clean': continue
+    #
+    test_sets = ["test-clean", "test-other"]
+    for test_set, test_dl in zip(test_sets, librispeech.test_dataloaders()):
+        results_dict = decode_dataset(
+            dl=test_dl,
+            params=params,
+            model=model,
+            HLG=HLG,
+            lexicon=lexicon,
+            G=G,
+            sos_id=sos_id,
+            eos_id=eos_id,
+        )
+
+        save_results(
+            params=params, test_set_name=test_set, results_dict=results_dict
+        )
+
+    logging.info("Done!")
+
+
+torch.set_num_threads(1)
+torch.set_num_interop_threads(1)
+
+if __name__ == "__main__":
+    main()

egs/librispeech/ASR/conformer_mmi_phone/embedding.py

@@ -0,0 +1,221 @@
+
+# This file is copied & modified from pytorch/torch/nn/modules/sparse.py
+# It modifies nn.Embedding
+import math
+from typing import Optional
+
+import torch
+import torch.nn as nn
+import torch.nn.functional as F
+from torch import Tensor
+from torch.nn import Parameter
+
+
+class Embedding(nn.Module):
+    r"""A simple lookup table that stores embeddings of a fixed dictionary and size.
+
+    This module is often used to store word embeddings and retrieve them using indices.
+    The input to the module is a list of indices, and the output is the corresponding
+    word embeddings.
+
+    Args:
+        num_embeddings (int): size of the dictionary of embeddings
+        embedding_dim (int): the size of each embedding vector
+        padding_idx (int, optional): If specified, the entries at :attr:`padding_idx` do not contribute to the gradient;
+                                     therefore, the embedding vector at :attr:`padding_idx` is not updated during training,
+                                     i.e. it remains as a fixed "pad". For a newly constructed Embedding,
+                                     the embedding vector at :attr:`padding_idx` will default to all zeros,
+                                     but can be updated to another value to be used as the padding vector.
+        max_norm (float, optional): If given, each embedding vector with norm larger than :attr:`max_norm`
+                                    is renormalized to have norm :attr:`max_norm`.
+        norm_type (float, optional): The p of the p-norm to compute for the :attr:`max_norm` option. Default ``2``.
+        scale_grad_by_freq (boolean, optional): If given, this will scale gradients by the inverse of frequency of
+                                                the words in the mini-batch. Default ``False``.
+        sparse (bool, optional): If ``True``, gradient w.r.t. :attr:`weight` matrix will be a sparse tensor.
+                                 See Notes for more details regarding sparse gradients.
+
+    Attributes:
+        weight (Tensor): the learnable weights of the module of shape (num_embeddings, embedding_dim)
+                         initialized from :math:`\mathcal{N}(0, 1)`
+
+    Shape:
+        - Input: :math:`(*)`, IntTensor or LongTensor of arbitrary shape containing the indices to extract
+        - Output: :math:`(*, H)`, where `*` is the input shape and :math:`H=\text{embedding\_dim}`
+
+    .. note::
+        Keep in mind that only a limited number of optimizers support
+        sparse gradients: currently it's :class:`optim.SGD` (`CUDA` and `CPU`),
+        :class:`optim.SparseAdam` (`CUDA` and `CPU`) and :class:`optim.Adagrad` (`CPU`)
+
+    .. note::
+        When :attr:`max_norm` is not ``None``, :class:`Embedding`'s forward method will modify the
+        :attr:`weight` tensor in-place. Since tensors needed for gradient computations cannot be
+        modified in-place, performing a differentiable operation on ``Embedding.weight`` before
+        calling :class:`Embedding`'s forward method requires cloning ``Embedding.weight`` when
+        :attr:`max_norm` is not ``None``. For example::
+
+            n, d, m = 3, 5, 7
+            embedding = nn.Embedding(n, d, max_norm=True)
+            W = torch.randn((m, d), requires_grad=True)
+            idx = torch.tensor([1, 2])
+            a = embedding.weight.clone() @ W.t()  # weight must be cloned for this to be differentiable
+            b = embedding(idx) @ W.t()  # modifies weight in-place
+            out = (a.unsqueeze(0) + b.unsqueeze(1))
+            loss = out.sigmoid().prod()
+            loss.backward()
+
+    Examples::
+
+        >>> # an Embedding module containing 10 tensors of size 3
+        >>> embedding = nn.Embedding(10, 3)
+        >>> # a batch of 2 samples of 4 indices each
+        >>> input = torch.LongTensor([[1,2,4,5],[4,3,2,9]])
+        >>> embedding(input)
+        tensor([[[-0.0251, -1.6902,  0.7172],
+                 [-0.6431,  0.0748,  0.6969],
+                 [ 1.4970,  1.3448, -0.9685],
+                 [-0.3677, -2.7265, -0.1685]],
+
+                [[ 1.4970,  1.3448, -0.9685],
+                 [ 0.4362, -0.4004,  0.9400],
+                 [-0.6431,  0.0748,  0.6969],
+                 [ 0.9124, -2.3616,  1.1151]]])
+
+
+        >>> # example with padding_idx
+        >>> embedding = nn.Embedding(10, 3, padding_idx=0)
+        >>> input = torch.LongTensor([[0,2,0,5]])
+        >>> embedding(input)
+        tensor([[[ 0.0000,  0.0000,  0.0000],
+                 [ 0.1535, -2.0309,  0.9315],
+                 [ 0.0000,  0.0000,  0.0000],
+                 [-0.1655,  0.9897,  0.0635]]])
+
+        >>> # example of changing `pad` vector
+        >>> padding_idx = 0
+        >>> embedding = nn.Embedding(3, 3, padding_idx=padding_idx)
+        >>> embedding.weight
+        Parameter containing:
+        tensor([[ 0.0000,  0.0000,  0.0000],
+                [-0.7895, -0.7089, -0.0364],
+                [ 0.6778,  0.5803,  0.2678]], requires_grad=True)
+        >>> with torch.no_grad():
+        ...     embedding.weight[padding_idx] = torch.ones(3)
+        >>> embedding.weight
+        Parameter containing:
+        tensor([[ 1.0000,  1.0000,  1.0000],
+                [-0.7895, -0.7089, -0.0364],
+                [ 0.6778,  0.5803,  0.2678]], requires_grad=True)
+    """
+    __constants__ = ['num_embeddings', 'embedding_dim', 'padding_idx', 'max_norm',
+                     'norm_type', 'scale_grad_by_freq', 'sparse']
+
+    num_embeddings: int
+    embedding_dim: int
+    padding_idx: Optional[int]
+    max_norm: Optional[float]
+    norm_type: float
+    scale_grad_by_freq: bool
+    weight: Tensor
+    sparse: bool
+
+    def __init__(self, num_embeddings: int, embedding_dim: int, padding_idx: Optional[int] = None,
+                 max_norm: Optional[float] = None, norm_type: float = 2., scale_grad_by_freq: bool = False,
+                 sparse: bool = False, _weight: Optional[Tensor] = None) -> None:
+        super(Embedding, self).__init__()
+        self.num_embeddings = num_embeddings
+        self.embedding_dim = embedding_dim
+        if padding_idx is not None:
+            if padding_idx > 0:
+                assert padding_idx < self.num_embeddings, 'Padding_idx must be within num_embeddings'
+            elif padding_idx < 0:
+                assert padding_idx >= -self.num_embeddings, 'Padding_idx must be within num_embeddings'
+                padding_idx = self.num_embeddings + padding_idx
+        self.padding_idx = padding_idx
+        self.max_norm = max_norm
+        self.norm_type = norm_type
+        self.scale_grad_by_freq = scale_grad_by_freq
+        self.embedding_scale = math.sqrt(self.embedding_dim)
+        if _weight is None:
+            self.weight = Parameter(torch.empty(num_embeddings, embedding_dim))
+            self.reset_parameters()
+        else:
+            assert list(_weight.shape) == [num_embeddings, embedding_dim], \
+                'Shape of weight does not match num_embeddings and embedding_dim'
+            self.weight = Parameter(_weight)
+
+        self.sparse = sparse
+
+    def reset_parameters(self) -> None:
+        std = 1 / self.embedding_scale
+        nn.init.normal_(self.weight, std=std)
+        self._fill_padding_idx_with_zero()
+
+    def _fill_padding_idx_with_zero(self) -> None:
+        if self.padding_idx is not None:
+            with torch.no_grad():
+                self.weight[self.padding_idx].fill_(0)
+
+    def forward(self, input: Tensor) -> Tensor:
+        return F.embedding(
+            input, self.weight, self.padding_idx, self.max_norm,
+            self.norm_type, self.scale_grad_by_freq, self.sparse) * self.embedding_scale
+
+    def extra_repr(self) -> str:
+        s = '{num_embeddings}, {embedding_dim}'
+        if self.padding_idx is not None:
+            s += ', padding_idx={padding_idx}'
+        if self.max_norm is not None:
+            s += ', max_norm={max_norm}'
+        if self.norm_type != 2:
+            s += ', norm_type={norm_type}'
+        if self.scale_grad_by_freq is not False:
+            s += ', scale_grad_by_freq={scale_grad_by_freq}'
+        if self.sparse is not False:
+            s += ', sparse=True'
+        return s.format(**self.__dict__)
+
+    @classmethod
+    def from_pretrained(cls, embeddings, freeze=True, padding_idx=None,
+                        max_norm=None, norm_type=2., scale_grad_by_freq=False,
+                        sparse=False):
+        r"""Creates Embedding instance from given 2-dimensional FloatTensor.
+
+        Args:
+            embeddings (Tensor): FloatTensor containing weights for the Embedding.
+                First dimension is being passed to Embedding as ``num_embeddings``, second as ``embedding_dim``.
+            freeze (boolean, optional): If ``True``, the tensor does not get updated in the learning process.
+                Equivalent to ``embedding.weight.requires_grad = False``. Default: ``True``
+            padding_idx (int, optional): If specified, the entries at :attr:`padding_idx` do not contribute to the gradient;
+                                         therefore, the embedding vector at :attr:`padding_idx` is not updated during training,
+                                         i.e. it remains as a fixed "pad".
+            max_norm (float, optional): See module initialization documentation.
+            norm_type (float, optional): See module initialization documentation. Default ``2``.
+            scale_grad_by_freq (boolean, optional): See module initialization documentation. Default ``False``.
+            sparse (bool, optional): See module initialization documentation.
+
+        Examples::
+
+            >>> # FloatTensor containing pretrained weights
+            >>> weight = torch.FloatTensor([[1, 2.3, 3], [4, 5.1, 6.3]])
+            >>> embedding = nn.Embedding.from_pretrained(weight)
+            >>> # Get embeddings for index 1
+            >>> input = torch.LongTensor([1])
+            >>> embedding(input)
+            tensor([[ 4.0000,  5.1000,  6.3000]])
+        """
+        assert embeddings.dim() == 2, \
+            'Embeddings parameter is expected to be 2-dimensional'
+        rows, cols = embeddings.shape
+        embedding = cls(
+            num_embeddings=rows,
+            embedding_dim=cols,
+            _weight=embeddings,
+            padding_idx=padding_idx,
+            max_norm=max_norm,
+            norm_type=norm_type,
+            scale_grad_by_freq=scale_grad_by_freq,
+            sparse=sparse)
+        embedding.weight.requires_grad = not freeze
+        return embedding
+

egs/librispeech/ASR/conformer_mmi_phone/pretrained.py

@@ -0,0 +1,350 @@
+#!/usr/bin/env python3
+
+import argparse
+import logging
+import math
+from typing import List
+
+import k2
+import kaldifeat
+import torch
+import torchaudio
+from conformer import Conformer
+from torch.nn.utils.rnn import pad_sequence
+
+from icefall.decode import (
+    get_lattice,
+    one_best_decoding,
+    rescore_with_attention_decoder,
+    rescore_with_whole_lattice,
+)
+from icefall.utils import AttributeDict, get_texts
+
+
+def get_parser():
+    parser = argparse.ArgumentParser(
+        formatter_class=argparse.ArgumentDefaultsHelpFormatter
+    )
+
+    parser.add_argument(
+        "--checkpoint",
+        type=str,
+        required=True,
+        help="Path to the checkpoint. "
+        "The checkpoint is assumed to be saved by "
+        "icefall.checkpoint.save_checkpoint().",
+    )
+
+    parser.add_argument(
+        "--words-file",
+        type=str,
+        required=True,
+        help="Path to words.txt",
+    )
+
+    parser.add_argument(
+        "--HLG", type=str, required=True, help="Path to HLG.pt."
+    )
+
+    parser.add_argument(
+        "--method",
+        type=str,
+        default="1best",
+        help="""Decoding method.
+        Possible values are:
+        (1) 1best - Use the best path as decoding output. Only
+            the transformer encoder output is used for decoding.
+            We call it HLG decoding.
+        (2) whole-lattice-rescoring - Use an LM to rescore the
+            decoding lattice and then use 1best to decode the
+            rescored lattice.
+            We call it HLG decoding + n-gram LM rescoring.
+        (3) attention-decoder - Extract n paths from he rescored
+            lattice and use the transformer attention decoder for
+            rescoring.
+            We call it HLG decoding + n-gram LM rescoring + attention
+            decoder rescoring.
+        """,
+    )
+
+    parser.add_argument(
+        "--G",
+        type=str,
+        help="""An LM for rescoring.
+        Used only when method is
+        whole-lattice-rescoring or attention-decoder.
+        It's usually a 4-gram LM.
+        """,
+    )
+
+    parser.add_argument(
+        "--num-paths",
+        type=int,
+        default=100,
+        help="""
+        Used only when method is attention-decoder.
+        It specifies the size of n-best list.""",
+    )
+
+    parser.add_argument(
+        "--ngram-lm-scale",
+        type=float,
+        default=1.3,
+        help="""
+        Used only when method is whole-lattice-rescoring and attention-decoder.
+        It specifies the scale for n-gram LM scores.
+        (Note: You need to tune it on a dataset.)
+        """,
+    )
+
+    parser.add_argument(
+        "--attention-decoder-scale",
+        type=float,
+        default=1.2,
+        help="""
+        Used only when method is attention-decoder.
+        It specifies the scale for attention decoder scores.
+        (Note: You need to tune it on a dataset.)
+        """,
+    )
+
+    parser.add_argument(
+        "--lattice-score-scale",
+        type=float,
+        default=0.5,
+        help="""
+        Used only when method is attention-decoder.
+        It specifies the scale for lattice.scores when
+        extracting n-best lists. A smaller value results in
+        more unique number of paths with the risk of missing
+        the best path.
+        """,
+    )
+
+    parser.add_argument(
+        "--sos-id",
+        type=float,
+        default=1,
+        help="""
+        Used only when method is attention-decoder.
+        It specifies ID for the SOS token.
+        """,
+    )
+
+    parser.add_argument(
+        "--eos-id",
+        type=float,
+        default=1,
+        help="""
+        Used only when method is attention-decoder.
+        It specifies ID for the EOS token.
+        """,
+    )
+
+    parser.add_argument(
+        "sound_files",
+        type=str,
+        nargs="+",
+        help="The input sound file(s) to transcribe. "
+        "Supported formats are those supported by torchaudio.load(). "
+        "For example, wav and flac are supported. "
+        "The sample rate has to be 16kHz.",
+    )
+
+    return parser
+
+
+def get_params() -> AttributeDict:
+    params = AttributeDict(
+        {
+            "feature_dim": 80,
+            "nhead": 8,
+            "num_classes": 5000,
+            "sample_rate": 16000,
+            "attention_dim": 512,
+            "subsampling_factor": 4,
+            "num_decoder_layers": 6,
+            "vgg_frontend": False,
+            "is_espnet_structure": True,
+            "mmi_loss": False,
+            "use_feat_batchnorm": True,
+            "search_beam": 20,
+            "output_beam": 8,
+            "min_active_states": 30,
+            "max_active_states": 10000,
+            "use_double_scores": True,
+        }
+    )
+    return params
+
+
+def read_sound_files(
+    filenames: List[str], expected_sample_rate: float
+) -> List[torch.Tensor]:
+    """Read a list of sound files into a list 1-D float32 torch tensors.
+    Args:
+      filenames:
+        A list of sound filenames.
+      expected_sample_rate:
+        The expected sample rate of the sound files.
+    Returns:
+      Return a list of 1-D float32 torch tensors.
+    """
+    ans = []
+    for f in filenames:
+        wave, sample_rate = torchaudio.load(f)
+        assert sample_rate == expected_sample_rate, (
+            f"expected sample rate: {expected_sample_rate}. "
+            f"Given: {sample_rate}"
+        )
+        # We use only the first channel
+        ans.append(wave[0])
+    return ans
+
+
+def main():
+    parser = get_parser()
+    args = parser.parse_args()
+
+    params = get_params()
+    params.update(vars(args))
+    logging.info(f"{params}")
+
+    device = torch.device("cpu")
+    if torch.cuda.is_available():
+        device = torch.device("cuda", 0)
+
+    logging.info(f"device: {device}")
+
+    logging.info("Creating model")
+    model = Conformer(
+        num_features=params.feature_dim,
+        nhead=params.nhead,
+        d_model=params.attention_dim,
+        num_classes=params.num_classes,
+        subsampling_factor=params.subsampling_factor,
+        num_decoder_layers=params.num_decoder_layers,
+        vgg_frontend=params.vgg_frontend,
+        is_espnet_structure=params.is_espnet_structure,
+        mmi_loss=params.mmi_loss,
+        use_feat_batchnorm=params.use_feat_batchnorm,
+    )
+
+    checkpoint = torch.load(args.checkpoint, map_location="cpu")
+    model.load_state_dict(checkpoint["model"])
+    model.to(device)
+    model.eval()
+
+    logging.info(f"Loading HLG from {params.HLG}")
+    HLG = k2.Fsa.from_dict(torch.load(params.HLG, map_location="cpu"))
+    HLG = HLG.to(device)
+    if not hasattr(HLG, "lm_scores"):
+        # For whole-lattice-rescoring and attention-decoder
+        HLG.lm_scores = HLG.scores.clone()
+
+    if params.method in ["whole-lattice-rescoring", "attention-decoder"]:
+        logging.info(f"Loading G from {params.G}")
+        G = k2.Fsa.from_dict(torch.load(params.G, map_location="cpu"))
+        G = G.to(device)
+        # Add epsilon self-loops to G as we will compose
+        # it with the whole lattice later
+        G = k2.add_epsilon_self_loops(G)
+        G = k2.arc_sort(G)
+        G.lm_scores = G.scores.clone()
+
+    logging.info("Constructing Fbank computer")
+    opts = kaldifeat.FbankOptions()
+    opts.device = device
+    opts.frame_opts.dither = 0
+    opts.frame_opts.snip_edges = False
+    opts.frame_opts.samp_freq = params.sample_rate
+    opts.mel_opts.num_bins = params.feature_dim
+
+    fbank = kaldifeat.Fbank(opts)
+
+    logging.info(f"Reading sound files: {params.sound_files}")
+    waves = read_sound_files(
+        filenames=params.sound_files, expected_sample_rate=params.sample_rate
+    )
+    waves = [w.to(device) for w in waves]
+
+    logging.info(f"Decoding started")
+    features = fbank(waves)
+
+    features = pad_sequence(
+        features, batch_first=True, padding_value=math.log(1e-10)
+    )
+
+    # Note: We don't use key padding mask for attention during decoding
+    with torch.no_grad():
+        nnet_output, memory, memory_key_padding_mask = model(features)
+
+    batch_size = nnet_output.shape[0]
+    supervision_segments = torch.tensor(
+        [[i, 0, nnet_output.shape[1]] for i in range(batch_size)],
+        dtype=torch.int32,
+    )
+
+    lattice = get_lattice(
+        nnet_output=nnet_output,
+        HLG=HLG,
+        supervision_segments=supervision_segments,
+        search_beam=params.search_beam,
+        output_beam=params.output_beam,
+        min_active_states=params.min_active_states,
+        max_active_states=params.max_active_states,
+        subsampling_factor=params.subsampling_factor,
+    )
+
+    if params.method == "1best":
+        logging.info("Use HLG decoding")
+        best_path = one_best_decoding(
+            lattice=lattice, use_double_scores=params.use_double_scores
+        )
+    elif params.method == "whole-lattice-rescoring":
+        logging.info("Use HLG decoding + LM rescoring")
+        best_path_dict = rescore_with_whole_lattice(
+            lattice=lattice,
+            G_with_epsilon_loops=G,
+            lm_scale_list=[params.ngram_lm_scale],
+        )
+        best_path = next(iter(best_path_dict.values()))
+    elif params.method == "attention-decoder":
+        logging.info("Use HLG + LM rescoring + attention decoder rescoring")
+        rescored_lattice = rescore_with_whole_lattice(
+            lattice=lattice, G_with_epsilon_loops=G, lm_scale_list=None
+        )
+        best_path_dict = rescore_with_attention_decoder(
+            lattice=rescored_lattice,
+            num_paths=params.num_paths,
+            model=model,
+            memory=memory,
+            memory_key_padding_mask=memory_key_padding_mask,
+            sos_id=params.sos_id,
+            eos_id=params.eos_id,
+            scale=params.lattice_score_scale,
+            ngram_lm_scale=params.ngram_lm_scale,
+            attention_scale=params.attention_decoder_scale,
+        )
+        best_path = next(iter(best_path_dict.values()))
+
+    hyps = get_texts(best_path)
+    word_sym_table = k2.SymbolTable.from_file(params.words_file)
+    hyps = [[word_sym_table[i] for i in ids] for ids in hyps]
+
+    s = "\n"
+    for filename, hyp in zip(params.sound_files, hyps):
+        words = " ".join(hyp)
+        s += f"{filename}:\n{words}\n\n"
+    logging.info(s)
+
+    logging.info(f"Decoding Done")
+
+
+if __name__ == "__main__":
+    formatter = (
+        "%(asctime)s %(levelname)s [%(filename)s:%(lineno)d] %(message)s"
+    )
+
+    logging.basicConfig(format=formatter, level=logging.INFO)
+    main()

egs/librispeech/ASR/conformer_mmi_phone/subsampling.py

@@ -0,0 +1,144 @@
+import torch
+import torch.nn as nn
+
+
+class Conv2dSubsampling(nn.Module):
+    """Convolutional 2D subsampling (to 1/4 length).
+
+    Convert an input of shape [N, T, idim] to an output
+    with shape [N, T', odim], where
+    T' = ((T-1)//2 - 1)//2, which approximates T' == T//4
+
+    It is based on
+    https://github.com/espnet/espnet/blob/master/espnet/nets/pytorch_backend/transformer/subsampling.py  # noqa
+    """
+
+    def __init__(self, idim: int, odim: int) -> None:
+        """
+        Args:
+          idim:
+            Input dim. The input shape is [N, T, idim].
+            Caution: It requires: T >=7, idim >=7
+          odim:
+            Output dim. The output shape is [N, ((T-1)//2 - 1)//2, odim]
+        """
+        assert idim >= 7
+        super().__init__()
+        self.conv = nn.Sequential(
+            nn.Conv2d(
+                in_channels=1, out_channels=odim, kernel_size=3, stride=2
+            ),
+            nn.ReLU(),
+            nn.Conv2d(
+                in_channels=odim, out_channels=odim, kernel_size=3, stride=2
+            ),
+            nn.ReLU(),
+        )
+        self.out = nn.Linear(odim * (((idim - 1) // 2 - 1) // 2), odim)
+
+    def forward(self, x: torch.Tensor) -> torch.Tensor:
+        """Subsample x.
+
+        Args:
+          x:
+            Its shape is [N, T, idim].
+
+        Returns:
+          Return a tensor of shape [N, ((T-1)//2 - 1)//2, odim]
+        """
+        # On entry, x is [N, T, idim]
+        x = x.unsqueeze(1)  # [N, T, idim] -> [N, 1, T, idim] i.e., [N, C, H, W]
+        x = self.conv(x)
+        # Now x is of shape [N, odim, ((T-1)//2 - 1)//2, ((idim-1)//2 - 1)//2]
+        b, c, t, f = x.size()
+        x = self.out(x.transpose(1, 2).contiguous().view(b, t, c * f))
+        # Now x is of shape [N, ((T-1)//2 - 1))//2, odim]
+        return x
+
+
+class VggSubsampling(nn.Module):
+    """Trying to follow the setup described in the following paper:
+    https://arxiv.org/pdf/1910.09799.pdf
+
+    This paper is not 100% explicit so I am guessing to some extent,
+    and trying to compare with other VGG implementations.
+
+    Convert an input of shape [N, T, idim] to an output
+    with shape [N, T', odim], where
+    T' = ((T-1)//2 - 1)//2, which approximates T' = T//4
+    """
+
+    def __init__(self, idim: int, odim: int) -> None:
+        """Construct a VggSubsampling object.
+
+        This uses 2 VGG blocks with 2 Conv2d layers each,
+        subsampling its input by a factor of 4 in the time dimensions.
+
+        Args:
+          idim:
+            Input dim. The input shape is [N, T, idim].
+            Caution: It requires: T >=7, idim >=7
+          odim:
+            Output dim. The output shape is [N, ((T-1)//2 - 1)//2, odim]
+        """
+        super().__init__()
+
+        cur_channels = 1
+        layers = []
+        block_dims = [32, 64]
+
+        # The decision to use padding=1 for the 1st convolution, then padding=0
+        # for the 2nd and for the max-pooling, and ceil_mode=True, was driven by
+        # a back-compatibility concern so that the number of frames at the
+        # output would be equal to:
+        #  (((T-1)//2)-1)//2.
+        # We can consider changing this by using padding=1 on the
+        # 2nd convolution, so the num-frames at the output would be T//4.
+        for block_dim in block_dims:
+            layers.append(
+                torch.nn.Conv2d(
+                    in_channels=cur_channels,
+                    out_channels=block_dim,
+                    kernel_size=3,
+                    padding=1,
+                    stride=1,
+                )
+            )
+            layers.append(torch.nn.ReLU())
+            layers.append(
+                torch.nn.Conv2d(
+                    in_channels=block_dim,
+                    out_channels=block_dim,
+                    kernel_size=3,
+                    padding=0,
+                    stride=1,
+                )
+            )
+            layers.append(
+                torch.nn.MaxPool2d(
+                    kernel_size=2, stride=2, padding=0, ceil_mode=True
+                )
+            )
+            cur_channels = block_dim
+
+        self.layers = nn.Sequential(*layers)
+
+        self.out = nn.Linear(
+            block_dims[-1] * (((idim - 1) // 2 - 1) // 2), odim
+        )
+
+    def forward(self, x: torch.Tensor) -> torch.Tensor:
+        """Subsample x.
+
+        Args:
+          x:
+            Its shape is [N, T, idim].
+
+        Returns:
+          Return a tensor of shape [N, ((T-1)//2 - 1)//2, odim]
+        """
+        x = x.unsqueeze(1)
+        x = self.layers(x)
+        b, c, t, f = x.size()
+        x = self.out(x.transpose(1, 2).contiguous().view(b, t, c * f))
+        return x

egs/librispeech/ASR/conformer_mmi_phone/test_subsampling.py

@@ -0,0 +1,33 @@
+#!/usr/bin/env python3
+
+from subsampling import Conv2dSubsampling
+from subsampling import VggSubsampling
+import torch
+
+
+def test_conv2d_subsampling():
+    N = 3
+    odim = 2
+
+    for T in range(7, 19):
+        for idim in range(7, 20):
+            model = Conv2dSubsampling(idim=idim, odim=odim)
+            x = torch.empty(N, T, idim)
+            y = model(x)
+            assert y.shape[0] == N
+            assert y.shape[1] == ((T - 1) // 2 - 1) // 2
+            assert y.shape[2] == odim
+
+
+def test_vgg_subsampling():
+    N = 3
+    odim = 2
+
+    for T in range(7, 19):
+        for idim in range(7, 20):
+            model = VggSubsampling(idim=idim, odim=odim)
+            x = torch.empty(N, T, idim)
+            y = model(x)
+            assert y.shape[0] == N
+            assert y.shape[1] == ((T - 1) // 2 - 1) // 2
+            assert y.shape[2] == odim

egs/librispeech/ASR/conformer_mmi_phone/test_transformer.py

@@ -0,0 +1,89 @@
+#!/usr/bin/env python3
+
+import torch
+from transformer import (
+    Transformer,
+    encoder_padding_mask,
+    generate_square_subsequent_mask,
+    decoder_padding_mask,
+    add_sos,
+    add_eos,
+)
+
+from torch.nn.utils.rnn import pad_sequence
+
+
+def test_encoder_padding_mask():
+    supervisions = {
+        "sequence_idx": torch.tensor([0, 1, 2]),
+        "start_frame": torch.tensor([0, 0, 0]),
+        "num_frames": torch.tensor([18, 7, 13]),
+    }
+
+    max_len = ((18 - 1) // 2 - 1) // 2
+    mask = encoder_padding_mask(max_len, supervisions)
+    expected_mask = torch.tensor(
+        [
+            [False, False, False],  # ((18 - 1)//2 - 1)//2 = 3,
+            [False, True, True],  # ((7 - 1)//2 - 1)//2 = 1,
+            [False, False, True],  # ((13 - 1)//2 - 1)//2 = 2,
+        ]
+    )
+    assert torch.all(torch.eq(mask, expected_mask))
+
+
+def test_transformer():
+    num_features = 40
+    num_classes = 87
+    model = Transformer(num_features=num_features, num_classes=num_classes)
+
+    N = 31
+
+    for T in range(7, 30):
+        x = torch.rand(N, T, num_features)
+        y, _, _ = model(x)
+        assert y.shape == (N, (((T - 1) // 2) - 1) // 2, num_classes)
+
+
+def test_generate_square_subsequent_mask():
+    s = 5
+    mask = generate_square_subsequent_mask(s)
+    inf = float("inf")
+    expected_mask = torch.tensor(
+        [
+            [0.0, -inf, -inf, -inf, -inf],
+            [0.0, 0.0, -inf, -inf, -inf],
+            [0.0, 0.0, 0.0, -inf, -inf],
+            [0.0, 0.0, 0.0, 0.0, -inf],
+            [0.0, 0.0, 0.0, 0.0, 0.0],
+        ]
+    )
+    assert torch.all(torch.eq(mask, expected_mask))
+
+
+def test_decoder_padding_mask():
+    x = [torch.tensor([1, 2]), torch.tensor([3]), torch.tensor([2, 5, 8])]
+    y = pad_sequence(x, batch_first=True, padding_value=-1)
+    mask = decoder_padding_mask(y, ignore_id=-1)
+    expected_mask = torch.tensor(
+        [
+            [False, False, True],
+            [False, True, True],
+            [False, False, False],
+        ]
+    )
+    assert torch.all(torch.eq(mask, expected_mask))
+
+
+def test_add_sos():
+    x = [[1, 2], [3], [2, 5, 8]]
+    y = add_sos(x, sos_id=0)
+    expected_y = [[0, 1, 2], [0, 3], [0, 2, 5, 8]]
+    assert y == expected_y
+
+
+def test_add_eos():
+    x = [[1, 2], [3], [2, 5, 8]]
+    y = add_eos(x, eos_id=0)
+    expected_y = [[1, 2, 0], [3, 0], [2, 5, 8, 0]]
+    assert y == expected_y

egs/librispeech/ASR/conformer_mmi_phone/train.py

@@ -0,0 +1,839 @@
+#!/usr/bin/env python3
+
+import argparse
+import gc
+import logging
+from pathlib import Path
+from shutil import copyfile
+from typing import Optional
+
+import k2
+import torch
+import torch.distributed as dist
+import torch.multiprocessing as mp
+import torch.nn as nn
+from asr_datamodule import LibriSpeechAsrDataModule
+from conformer import Conformer
+from lhotse.utils import fix_random_seed
+from tdnn_lstm_ctc.model import TdnnLstm
+from torch.nn.parallel import DistributedDataParallel as DDP
+from torch.nn.utils import clip_grad_norm_
+from torch.utils.tensorboard import SummaryWriter
+from transformer import Noam
+
+from icefall.checkpoint import load_checkpoint
+from icefall.checkpoint import save_checkpoint as save_checkpoint_impl
+from icefall.dist import cleanup_dist, setup_dist
+from icefall.lexicon import Lexicon
+from icefall.mmi import LFMMILoss
+from icefall.mmi_graph_compiler import MmiTrainingGraphCompiler
+from icefall.utils import (
+    AttributeDict,
+    encode_supervisions,
+    setup_logger,
+    str2bool,
+)
+
+
+def get_parser():
+    parser = argparse.ArgumentParser(
+        formatter_class=argparse.ArgumentDefaultsHelpFormatter
+    )
+
+    parser.add_argument(
+        "--world-size",
+        type=int,
+        default=1,
+        help="Number of GPUs for DDP training.",
+    )
+
+    parser.add_argument(
+        "--master-port",
+        type=int,
+        default=12354,
+        help="Master port to use for DDP training.",
+    )
+
+    parser.add_argument(
+        "--tensorboard",
+        type=str2bool,
+        default=True,
+        help="Should various information be logged in tensorboard.",
+    )
+
+    parser.add_argument(
+        "--use-ali-model",
+        type=str2bool,
+        default=False,
+        help="If true, we assume that you have run tdnn_lstm_ctc/train_bpe.py "
+        "and you have some checkpoints inside the directory "
+        "tdnn_lstm_ctc/exp_bpe_500 ."
+        "It will use tdnn_lstm_ctc/exp_bpe_500/epoch-{ali-model-epoch}.pt "
+        "as the pre-trained alignment model",
+    )
+    parser.add_argument(
+        "--ali-model-epoch",
+        type=int,
+        default=19,
+        help="If --use-ali-model is True, load "
+        "tdnn_lstm_ctc/exp_bpe_500/epoch-{ali-model-epoch}.pt as "
+        "the alignment model."
+        "Used only if --use-ali-model is True.",
+    )
+
+    # TODO: add extra arguments and support DDP training.
+    # Currently, only single GPU training is implemented. Will add
+    # DDP training once single GPU training is finished.
+    return parser
+
+
+def get_params() -> AttributeDict:
+    """Return a dict containing training parameters.
+
+    All training related parameters that are not passed from the commandline
+    is saved in the variable `params`.
+
+    Commandline options are merged into `params` after they are parsed, so
+    you can also access them via `params`.
+
+    Explanation of options saved in `params`:
+
+        - exp_dir: It specifies the directory where all training related
+                   files, e.g., checkpoints, log, etc, are saved
+
+        - lang_dir: It contains language related input files such as
+                    "lexicon.txt"
+
+        - lr: It specifies the initial learning rate
+
+        - feature_dim: The model input dim. It has to match the one used
+                       in computing features.
+
+        - weight_decay:  The weight_decay for the optimizer.
+
+        - subsampling_factor:  The subsampling factor for the model.
+
+        - start_epoch:  If it is not zero, load checkpoint `start_epoch-1`
+                        and continue training from that checkpoint.
+
+        - num_epochs:  Number of epochs to train.
+
+        - best_train_loss: Best training loss so far. It is used to select
+                           the model that has the lowest training loss. It is
+                           updated during the training.
+
+        - best_valid_loss: Best validation loss so far. It is used to select
+                           the model that has the lowest validation loss. It is
+                           updated during the training.
+
+        - best_train_epoch: It is the epoch that has the best training loss.
+
+        - best_valid_epoch: It is the epoch that has the best validation loss.
+
+        - batch_idx_train: Used to writing statistics to tensorboard. It
+                           contains number of batches trained so far across
+                           epochs.
+
+        - log_interval:  Print training loss if batch_idx % log_interval` is 0
+
+        - valid_interval:  Run validation if batch_idx % valid_interval` is 0
+    """
+    params = AttributeDict(
+        {
+            "exp_dir": Path("conformer_mmi_phone/exp"),
+            "lang_dir": Path("data/lang_phone"),
+            "feature_dim": 80,
+            "weight_decay": 1e-6,
+            "subsampling_factor": 4,
+            "start_epoch": 0,
+            "num_epochs": 50,
+            "best_train_loss": float("inf"),
+            "best_valid_loss": float("inf"),
+            "best_train_epoch": -1,
+            "best_valid_epoch": -1,
+            "batch_idx_train": 0,
+            "log_interval": 50,
+            "reset_interval": 200,
+            "valid_interval": 3000,
+            "use_pruned_intersect": False,
+            "den_scale": 1.0,
+            #
+            "att_rate": 0,
+            "attention_dim": 512,
+            "nhead": 8,
+            "num_decoder_layers": 6,
+            "is_espnet_structure": True,
+            "use_feat_batchnorm": True,
+            "lr_factor": 5.0,
+            "warm_step": 80000,
+            #  "warm_step": 10000,
+        }
+    )
+
+    return params
+
+
+def load_checkpoint_if_available(
+    params: AttributeDict,
+    model: nn.Module,
+    optimizer: Optional[torch.optim.Optimizer] = None,
+    scheduler: Optional[torch.optim.lr_scheduler._LRScheduler] = None,
+) -> None:
+    """Load checkpoint from file.
+
+    If params.start_epoch is positive, it will load the checkpoint from
+    `params.start_epoch - 1`. Otherwise, this function does nothing.
+
+    Apart from loading state dict for `model`, `optimizer` and `scheduler`,
+    it also updates `best_train_epoch`, `best_train_loss`, `best_valid_epoch`,
+    and `best_valid_loss` in `params`.
+
+    Args:
+      params:
+        The return value of :func:`get_params`.
+      model:
+        The training model.
+      optimizer:
+        The optimizer that we are using.
+      scheduler:
+        The learning rate scheduler we are using.
+    Returns:
+      Return None.
+    """
+    if params.start_epoch <= 0:
+        return
+
+    filename = params.exp_dir / f"epoch-{params.start_epoch-1}.pt"
+    saved_params = load_checkpoint(
+        filename,
+        model=model,
+        optimizer=optimizer,
+        scheduler=scheduler,
+    )
+
+    keys = [
+        "best_train_epoch",
+        "best_valid_epoch",
+        "batch_idx_train",
+        "best_train_loss",
+        "best_valid_loss",
+    ]
+    for k in keys:
+        params[k] = saved_params[k]
+
+    return saved_params
+
+
+def save_checkpoint(
+    params: AttributeDict,
+    model: nn.Module,
+    optimizer: Optional[torch.optim.Optimizer] = None,
+    scheduler: Optional[torch.optim.lr_scheduler._LRScheduler] = None,
+    rank: int = 0,
+) -> None:
+    """Save model, optimizer, scheduler and training stats to file.
+
+    Args:
+      params:
+        It is returned by :func:`get_params`.
+      model:
+        The training model.
+    """
+    if rank != 0:
+        return
+    filename = params.exp_dir / f"epoch-{params.cur_epoch}.pt"
+    save_checkpoint_impl(
+        filename=filename,
+        model=model,
+        params=params,
+        optimizer=optimizer,
+        scheduler=scheduler,
+        rank=rank,
+    )
+
+    if params.best_train_epoch == params.cur_epoch:
+        best_train_filename = params.exp_dir / "best-train-loss.pt"
+        copyfile(src=filename, dst=best_train_filename)
+
+    if params.best_valid_epoch == params.cur_epoch:
+        best_valid_filename = params.exp_dir / "best-valid-loss.pt"
+        copyfile(src=filename, dst=best_valid_filename)
+
+
+def compute_loss_impl(
+    params: AttributeDict,
+    model: nn.Module,
+    ali_model: Optional[nn.Module],
+    batch: dict,
+    graph_compiler: MmiTrainingGraphCompiler,
+    is_training: bool,
+):
+    """
+    Compute MMI loss given the model and its inputs.
+
+    Args:
+      params:
+        Parameters for training. See :func:`get_params`.
+      model:
+        The model for training. It is an instance of Conformer in our case.
+      batch:
+        A batch of data. See `lhotse.dataset.K2SpeechRecognitionDataset()`
+        for the content in it.
+      graph_compiler:
+        It is used to build num_graphs and den_graphs.
+      is_training:
+        True for training. False for validation. When it is True, this
+        function enables autograd during computation; when it is False, it
+        disables autograd.
+    """
+    device = graph_compiler.device
+    feature = batch["inputs"]
+    # at entry, feature is [N, T, C]
+    assert feature.ndim == 3
+    feature = feature.to(device)
+
+    supervisions = batch["supervisions"]
+    with torch.set_grad_enabled(is_training):
+        nnet_output, encoder_memory, memory_mask = model(feature, supervisions)
+        # nnet_output is [N, T, C]
+        if ali_model is not None and params.batch_idx_train < 4000:
+            feature = feature.permute(0, 2, 1)  # [N, T, C]->[N, C, T]
+            ali_model_output = ali_model(feature)
+            # subsampling is done slightly differently, may be small length
+            # differences.
+            min_len = min(ali_model_output.shape[1], nnet_output.shape[1])
+            # scale less than one so it will be encouraged
+            # to mimic ali_model's output
+            ali_model_scale = 500.0 / (params.batch_idx_train + 500)
+
+            # Use clone() here or log-softmax backprop will fail.
+            nnet_output = nnet_output.clone()
+
+            nnet_output[:, :min_len, :] += (
+                ali_model_scale * ali_model_output[:, :min_len, :]
+            )
+
+    # NOTE: We need `encode_supervisions` to sort sequences with
+    # different duration in decreasing order, required by
+    # `k2.intersect_dense` called in LFMMILoss
+    #
+    # TODO: If params.use_pruned_intersect is True, there is no
+    # need to call encode_supervisions
+    supervision_segments, texts = encode_supervisions(
+        supervisions, subsampling_factor=params.subsampling_factor
+    )
+
+    dense_fsa_vec = k2.DenseFsaVec(
+        nnet_output,
+        supervision_segments,
+        allow_truncate=params.subsampling_factor - 1,
+    )
+
+    loss_fn = LFMMILoss(
+        graph_compiler=graph_compiler,
+        den_scale=params.den_scale,
+        use_pruned_intersect=params.use_pruned_intersect,
+    )
+
+    mmi_loss = loss_fn(dense_fsa_vec=dense_fsa_vec, texts=texts)
+
+    assert params.att_rate == 0
+    if params.att_rate != 0.0:
+        # TODO: not working
+        token_ids = graph_compiler.texts_to_ids(texts)
+        with torch.set_grad_enabled(is_training):
+            if hasattr(model, "module"):
+                att_loss = model.module.decoder_forward(
+                    encoder_memory,
+                    memory_mask,
+                    token_ids=token_ids,
+                    sos_id=graph_compiler.sos_id,
+                    eos_id=graph_compiler.eos_id,
+                )
+            else:
+                att_loss = model.decoder_forward(
+                    encoder_memory,
+                    memory_mask,
+                    token_ids=token_ids,
+                    sos_id=graph_compiler.sos_id,
+                    eos_id=graph_compiler.eos_id,
+                )
+        loss = (1.0 - params.att_rate) * mmi_loss + params.att_rate * att_loss
+    else:
+        loss = mmi_loss
+        att_loss = torch.tensor([0])
+
+    # train_frames and valid_frames are used for printing.
+    if is_training:
+        params.train_frames = supervision_segments[:, 2].sum().item()
+    else:
+        params.valid_frames = supervision_segments[:, 2].sum().item()
+
+    assert loss.requires_grad == is_training
+
+    return loss, mmi_loss.detach(), att_loss.detach()
+
+
+def compute_loss(
+    params: AttributeDict,
+    model: nn.Module,
+    ali_model: Optional[nn.Module],
+    batch: dict,
+    graph_compiler: MmiTrainingGraphCompiler,
+    is_training: bool,
+):
+    try:
+        return compute_loss_impl(
+            params=params,
+            model=model,
+            ali_model=ali_model,
+            batch=batch,
+            graph_compiler=graph_compiler,
+            is_training=is_training,
+        )
+    except RuntimeError as ex:
+        if "out of memory" not in str(ex):
+            raise ex
+
+        logging.exception(ex)
+        s = f"\nCaught exception: {str(ex)}\n"
+        total_duration = 0.0
+        max_cut_duration = 0.0
+        for cut in batch["supervisions"]["cut"]:
+            s += f" id: {cut.id}, duration: {cut.duration} seconds\n"
+            total_duration += cut.duration
+            max_cut_duration = max(max_cut_duration, cut.duration)
+        s += f" total duration: {total_duration:.3f} s\n"
+        s += f" max duration: {max_cut_duration:.3f} s \n"
+        logging.info(s)
+
+    torch.cuda.empty_cache()
+
+    gc.collect()
+
+    # See https://github.com/pytorch/pytorch/issues/18853#issuecomment-583779161
+    return compute_loss_impl(
+        params=params,
+        model=model,
+        ali_model=ali_model,
+        batch=params.saved_batch,
+        graph_compiler=graph_compiler,
+        is_training=is_training,
+    )
+
+
+def compute_validation_loss(
+    params: AttributeDict,
+    model: nn.Module,
+    ali_model: Optional[nn.Module],
+    graph_compiler: MmiTrainingGraphCompiler,
+    valid_dl: torch.utils.data.DataLoader,
+    world_size: int = 1,
+) -> None:
+    """Run the validation process. The validation loss
+    is saved in `params.valid_loss`.
+    """
+    model.eval()
+
+    tot_loss = 0.0
+    tot_mmi_loss = 0.0
+    tot_att_loss = 0.0
+    tot_frames = 0.0
+    for batch_idx, batch in enumerate(valid_dl):
+        loss, mmi_loss, att_loss = compute_loss(
+            params=params,
+            model=model,
+            ali_model=ali_model,
+            batch=batch,
+            graph_compiler=graph_compiler,
+            is_training=False,
+        )
+        assert loss.requires_grad is False
+        assert mmi_loss.requires_grad is False
+        assert att_loss.requires_grad is False
+
+        loss_cpu = loss.detach().cpu().item()
+        tot_loss += loss_cpu
+
+        tot_mmi_loss += mmi_loss.detach().cpu().item()
+        tot_att_loss += att_loss.detach().cpu().item()
+
+        tot_frames += params.valid_frames
+
+    if world_size > 1:
+        s = torch.tensor(
+            [tot_loss, tot_mmi_loss, tot_att_loss, tot_frames],
+            device=loss.device,
+        )
+        dist.all_reduce(s, op=dist.ReduceOp.SUM)
+        s = s.cpu().tolist()
+        tot_loss = s[0]
+        tot_mmi_loss = s[1]
+        tot_att_loss = s[2]
+        tot_frames = s[3]
+
+    params.valid_loss = tot_loss / tot_frames
+    params.valid_mmi_loss = tot_mmi_loss / tot_frames
+    params.valid_att_loss = tot_att_loss / tot_frames
+
+    if params.valid_loss < params.best_valid_loss:
+        params.best_valid_epoch = params.cur_epoch
+        params.best_valid_loss = params.valid_loss
+
+
+def train_one_epoch(
+    params: AttributeDict,
+    model: nn.Module,
+    ali_model: Optional[nn.Module],
+    optimizer: torch.optim.Optimizer,
+    graph_compiler: MmiTrainingGraphCompiler,
+    train_dl: torch.utils.data.DataLoader,
+    valid_dl: torch.utils.data.DataLoader,
+    tb_writer: Optional[SummaryWriter] = None,
+    world_size: int = 1,
+) -> None:
+    """Train the model for one epoch.
+
+    The training loss from the mean of all frames is saved in
+    `params.train_loss`. It runs the validation process every
+    `params.valid_interval` batches.
+
+    Args:
+      params:
+        It is returned by :func:`get_params`.
+      model:
+        The model for training.
+      ali_model:
+        The force alignment model for training. It is from
+        tdnn_lstm_ctc/train_bpe.py
+      optimizer:
+        The optimizer we are using.
+      graph_compiler:
+        It is used to convert transcripts to FSAs.
+      train_dl:
+        Dataloader for the training dataset.
+      valid_dl:
+        Dataloader for the validation dataset.
+      tb_writer:
+        Writer to write log messages to tensorboard.
+      world_size:
+        Number of nodes in DDP training. If it is 1, DDP is disabled.
+    """
+    model.train()
+
+    tot_loss = 0.0  # sum of losses over all batches
+    tot_mmi_loss = 0.0
+    tot_att_loss = 0.0
+
+    tot_frames = 0.0  # sum of frames over all batches
+    params.tot_loss = 0.0
+    params.tot_frames = 0.0
+    for batch_idx, batch in enumerate(train_dl):
+        if batch_idx == 0:
+            logging.info("save a batch for OOM handling")
+            # Use this batch to replace the batch that's causing OOM
+            params.saved_batch = batch
+
+        params.batch_idx_train += 1
+        batch_size = len(batch["supervisions"]["text"])
+
+        loss, mmi_loss, att_loss = compute_loss(
+            params=params,
+            model=model,
+            ali_model=ali_model,
+            batch=batch,
+            graph_compiler=graph_compiler,
+            is_training=True,
+        )
+
+        # NOTE: We use reduction==sum and loss is computed over utterances
+        # in the batch and there is no normalization to it so far.
+
+        optimizer.zero_grad()
+        loss.backward()
+        clip_grad_norm_(model.parameters(), max_norm=5.0, norm_type=2.0)
+        optimizer.step()
+
+        loss_cpu = loss.detach().cpu().item()
+        mmi_loss_cpu = mmi_loss.detach().cpu().item()
+        att_loss_cpu = att_loss.detach().cpu().item()
+
+        tot_frames += params.train_frames
+        tot_loss += loss_cpu
+        tot_mmi_loss += mmi_loss_cpu
+        tot_att_loss += att_loss_cpu
+
+        params.tot_frames += params.train_frames
+        params.tot_loss += loss_cpu
+
+        tot_avg_loss = tot_loss / tot_frames
+        tot_avg_mmi_loss = tot_mmi_loss / tot_frames
+        tot_avg_att_loss = tot_att_loss / tot_frames
+
+        if batch_idx % params.log_interval == 0:
+            total_duration = 0.0
+            max_cut_duration = 0.0
+            for cut in batch["supervisions"]["cut"]:
+                total_duration += cut.duration
+                max_cut_duration = max(max_cut_duration, cut.duration)
+            logging.info(
+                f"Epoch {params.cur_epoch}, batch {batch_idx}, "
+                f"batch avg mmi loss {mmi_loss_cpu/params.train_frames:.4f}, "
+                f"batch avg att loss {att_loss_cpu/params.train_frames:.4f}, "
+                f"batch avg loss {loss_cpu/params.train_frames:.4f}, "
+                f"total avg mmi loss: {tot_avg_mmi_loss:.4f}, "
+                f"total avg att loss: {tot_avg_att_loss:.4f}, "
+                f"total avg loss: {tot_avg_loss:.4f}, "
+                f"batch size: {batch_size}, "
+                f"total duration: {total_duration:.3f} s, "
+                f"max cut duration: {max_cut_duration:.3f} s"
+            )
+
+            if tb_writer is not None:
+                tb_writer.add_scalar(
+                    "train/current_mmi_loss",
+                    mmi_loss_cpu / params.train_frames,
+                    params.batch_idx_train,
+                )
+                tb_writer.add_scalar(
+                    "train/current_att_loss",
+                    att_loss_cpu / params.train_frames,
+                    params.batch_idx_train,
+                )
+                tb_writer.add_scalar(
+                    "train/current_loss",
+                    loss_cpu / params.train_frames,
+                    params.batch_idx_train,
+                )
+                tb_writer.add_scalar(
+                    "train/tot_avg_mmi_loss",
+                    tot_avg_mmi_loss,
+                    params.batch_idx_train,
+                )
+
+                tb_writer.add_scalar(
+                    "train/tot_avg_att_loss",
+                    tot_avg_att_loss,
+                    params.batch_idx_train,
+                )
+                tb_writer.add_scalar(
+                    "train/tot_avg_loss",
+                    tot_avg_loss,
+                    params.batch_idx_train,
+                )
+        if batch_idx > 0 and batch_idx % params.reset_interval == 0:
+            tot_loss = 0.0  # sum of losses over all batches
+            tot_mmi_loss = 0.0
+            tot_att_loss = 0.0
+
+            tot_frames = 0.0  # sum of frames over all batches
+
+        if batch_idx > 0 and batch_idx % params.valid_interval == 0:
+            compute_validation_loss(
+                params=params,
+                model=model,
+                ali_model=ali_model,
+                graph_compiler=graph_compiler,
+                valid_dl=valid_dl,
+                world_size=world_size,
+            )
+            model.train()
+            logging.info(
+                f"Epoch {params.cur_epoch}, "
+                f"valid mmi loss {params.valid_mmi_loss:.4f}, "
+                f"valid att loss {params.valid_att_loss:.4f}, "
+                f"valid loss {params.valid_loss:.4f}, "
+                f"best valid loss: {params.best_valid_loss:.4f}, "
+                f"best valid epoch: {params.best_valid_epoch}"
+            )
+            if tb_writer is not None:
+                tb_writer.add_scalar(
+                    "train/valid_mmi_loss",
+                    params.valid_mmi_loss,
+                    params.batch_idx_train,
+                )
+                tb_writer.add_scalar(
+                    "train/valid_att_loss",
+                    params.valid_att_loss,
+                    params.batch_idx_train,
+                )
+                tb_writer.add_scalar(
+                    "train/valid_loss",
+                    params.valid_loss,
+                    params.batch_idx_train,
+                )
+
+    params.train_loss = params.tot_loss / params.tot_frames
+
+    if params.train_loss < params.best_train_loss:
+        params.best_train_epoch = params.cur_epoch
+        params.best_train_loss = params.train_loss
+
+    if "saved_batch" in params:
+        del params["saved_batch"]
+
+
+def run(rank, world_size, args):
+    """
+    Args:
+      rank:
+        It is a value between 0 and `world_size-1`, which is
+        passed automatically by `mp.spawn()` in :func:`main`.
+        The node with rank 0 is responsible for saving checkpoint.
+      world_size:
+        Number of GPUs for DDP training.
+      args:
+        The return value of get_parser().parse_args()
+    """
+    params = get_params()
+    params.update(vars(args))
+
+    fix_random_seed(42)
+    if world_size > 1:
+        setup_dist(rank, world_size, params.master_port)
+
+    setup_logger(f"{params.exp_dir}/log/log-train")
+    logging.info("Training started")
+    logging.info(params)
+
+    if args.tensorboard and rank == 0:
+        tb_writer = SummaryWriter(log_dir=f"{params.exp_dir}/tensorboard")
+    else:
+        tb_writer = None
+
+    lexicon = Lexicon(params.lang_dir)
+    max_token_id = max(lexicon.tokens)
+    num_classes = max_token_id + 1  # +1 for the blank
+
+    device = torch.device("cpu")
+    if torch.cuda.is_available():
+        device = torch.device("cuda", rank)
+
+    graph_compiler = MmiTrainingGraphCompiler(
+        params.lang_dir,
+        device=device,
+        oov="<UNK>",
+    )
+
+    logging.info("About to create model")
+    model = Conformer(
+        num_features=params.feature_dim,
+        nhead=params.nhead,
+        d_model=params.attention_dim,
+        num_classes=num_classes,
+        subsampling_factor=params.subsampling_factor,
+        num_decoder_layers=params.num_decoder_layers,
+        vgg_frontend=False,
+        is_espnet_structure=params.is_espnet_structure,
+        use_feat_batchnorm=params.use_feat_batchnorm,
+    )
+
+    checkpoints = load_checkpoint_if_available(params=params, model=model)
+
+    model.to(device)
+    if world_size > 1:
+        model = DDP(model, device_ids=[rank])
+
+    optimizer = Noam(
+        model.parameters(),
+        model_size=params.attention_dim,
+        factor=params.lr_factor,
+        warm_step=params.warm_step,
+        weight_decay=params.weight_decay,
+    )
+
+    if checkpoints and checkpoints["optimizer"]:
+        optimizer.load_state_dict(checkpoints["optimizer"])
+
+    assert args.use_ali_model is False
+    if args.use_ali_model:
+        ali_model = TdnnLstm(
+            num_features=params.feature_dim,
+            num_classes=num_classes,
+            subsampling_factor=params.subsampling_factor,
+        )
+
+        # TODO: add an option to switch among
+        # bpe_500, bpe_1000, and bpe_5000
+        ali_model_fname = Path(
+            f"tdnn_lstm_ctc/exp_bpe_500/epoch-{args.ali_model_epoch}.pt"
+        )
+        assert (
+            ali_model_fname.is_file()
+        ), f"ali model filename {ali_model_fname} does not exist!"
+
+        ali_model.load_state_dict(
+            torch.load(ali_model_fname, map_location="cpu")["model"]
+        )
+        ali_model.to(device)
+
+        ali_model.eval()
+        ali_model.requires_grad_(False)
+        logging.info(f"Use ali_model: {ali_model_fname}")
+    else:
+        ali_model = None
+        logging.info("No ali_model")
+
+    librispeech = LibriSpeechAsrDataModule(args)
+    train_dl = librispeech.train_dataloaders()
+    valid_dl = librispeech.valid_dataloaders()
+
+    for epoch in range(params.start_epoch, params.num_epochs):
+        train_dl.sampler.set_epoch(epoch)
+
+        cur_lr = optimizer._rate
+        if tb_writer is not None:
+            tb_writer.add_scalar(
+                "train/learning_rate", cur_lr, params.batch_idx_train
+            )
+            tb_writer.add_scalar("train/epoch", epoch, params.batch_idx_train)
+
+        if rank == 0:
+            logging.info("epoch {}, learning rate {}".format(epoch, cur_lr))
+
+        params.cur_epoch = epoch
+
+        train_one_epoch(
+            params=params,
+            model=model,
+            ali_model=ali_model,
+            optimizer=optimizer,
+            graph_compiler=graph_compiler,
+            train_dl=train_dl,
+            valid_dl=valid_dl,
+            tb_writer=tb_writer,
+            world_size=world_size,
+        )
+
+        save_checkpoint(
+            params=params,
+            model=model,
+            optimizer=optimizer,
+            rank=rank,
+        )
+
+    logging.info("Done!")
+
+    if world_size > 1:
+        torch.distributed.barrier()
+        cleanup_dist()
+
+
+def main():
+    parser = get_parser()
+    LibriSpeechAsrDataModule.add_arguments(parser)
+    args = parser.parse_args()
+
+    world_size = args.world_size
+    assert world_size >= 1
+    if world_size > 1:
+        mp.spawn(run, args=(world_size, args), nprocs=world_size, join=True)
+    else:
+        run(rank=0, world_size=1, args=args)
+
+
+torch.set_num_threads(1)
+torch.set_num_interop_threads(1)
+
+if __name__ == "__main__":
+    main()

egs/librispeech/ASR/local/add_silence_to_transcript.py

@@ -0,0 +1,110 @@
+#!/usr/bin/env python3
+
+# Copyright 2021 Xiaomi Corporation (Author: Fangjun Kuang)
+'''
+Add silence with a given probability after each word in the transcript.
+
+If the input transcript contains:
+
+    hello world
+    foo bar koo
+    zoo
+
+Then the output transcript **may** look like the following:
+
+    !SIL hello !SIL world !SIL
+    foo bar !SIL koo !SIL
+    !SIL zoo !SIL
+
+(Assume !SIL represents silence.)
+'''
+
+from pathlib import Path
+
+import argparse
+import random
+
+
+def get_args():
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--transcript',
+                        type=str,
+                        help='The input transcript file.'
+                        'We assume that the transcript file consists of '
+                        'lines. Each line consists of space separated words.')
+    parser.add_argument('--sil-word',
+                        type=str,
+                        default='!SIL',
+                        help='The word that represents silence.')
+    parser.add_argument('--sil-prob',
+                        type=float,
+                        default=0.5,
+                        help='The probability for adding a '
+                        'silence after each world.')
+    parser.add_argument('--seed',
+                        type=int,
+                        default=None,
+                        help='The seed for random number generators.')
+
+    return parser.parse_args()
+
+
+def need_silence(sil_prob: float) -> bool:
+    '''
+    Args:
+      sil_prob:
+        The probability to add a silence.
+    Returns:
+      Return True if a silence is needed.
+      Return False otherwise.
+    '''
+    return random.uniform(0, 1) <= sil_prob
+
+
+def process_line(line: str, sil_word: str, sil_prob: float) -> None:
+    '''Process a single line from the transcript.
+
+    Args:
+      line:
+        A str containing space separated words.
+      sil_word:
+        The symbol indicating silence.
+      sil_prob:
+        The probability for adding a silence after each word.
+    Returns:
+      Return None.
+    '''
+    words = line.strip().split()
+    for i, word in enumerate(words):
+        if i == 0:
+            # beginning of the line
+            if need_silence(sil_prob):
+                print(sil_word, end=' ')
+
+        print(word, end=' ')
+
+        if need_silence(sil_prob):
+            print(sil_word, end=' ')
+
+        # end of the line, print a new line
+        if i == len(words) - 1:
+            print()
+
+
+def main():
+    args = get_args()
+    random.seed(args.seed)
+
+    assert Path(args.transcript).is_file()
+    assert len(args.sil_word) > 0
+    assert 0 < args.sil_prob < 1
+
+    with open(args.transcript) as f:
+        for line in f:
+            process_line(line=line,
+                         sil_word=args.sil_word,
+                         sil_prob=args.sil_prob)
+
+
+if __name__ == '__main__':
+    main()

egs/librispeech/ASR/local/convert_transcript_to_corpus.py

@@ -0,0 +1,104 @@
+#!/usr/bin/env python3
+
+# Copyright 2021 Xiaomi Corporation (Author: Fangjun Kuang)
+"""
+Convert a transcript file containing words to a corpus file containing tokens
+for LM training with the help of a lexicon.
+
+If the lexicon contains phones, the resulting LM will be a phone LM; If the
+lexicon contains word pieces, the resulting LM will be a word piece LM.
+
+If a word has multiple pronunciations, the one that appears last in the lexicon
+is used.
+
+If the input transcript is:
+
+    hello zoo world hello
+    world zoo
+    foo zoo world hellO
+
+and if the lexicon is
+
+    <UNK> SPN
+    hello h e l l o
+    hello h e l l o 2
+    world w o r l d
+    zoo z o o
+
+Then the output is
+
+    h e l l o 2 z o o w o r l d h e l l o 2
+    w o r l d z o o
+    SPN z o o w o r l d SPN
+"""
+
+from pathlib import Path
+from typing import Dict
+
+import argparse
+
+from icefall.lexicon import read_lexicon
+
+
+def get_args():
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        "--transcript",
+        type=str,
+        help="The input transcript file."
+        "We assume that the transcript file consists of "
+        "lines. Each line consists of space separated words.",
+    )
+    parser.add_argument("--lexicon", type=str, help="The input lexicon file.")
+    parser.add_argument(
+        "--oov", type=str, default="<UNK>", help="The OOV word."
+    )
+
+    return parser.parse_args()
+
+
+def process_line(
+    lexicon: Dict[str, List[str]], line: str, oov_token: str
+) -> None:
+    """
+    Args:
+      lexicon:
+        A dict containing pronunciations. Its keys are words and values
+        are pronunciations (i.e., tokens).
+      line:
+        A line of transcript consisting of space(s) separated words.
+      oov_token:
+        The pronunciation of the oov word if a word in `line` is not present
+        in the lexicon.
+    Returns:
+      Return None.
+    """
+    s = ""
+    words = line.strip().split()
+    for i, w in enumerate(words):
+        tokens = lexicon.get(w, oov_token)
+        s += " ".join(tokens)
+        s += " "
+    print(s.strip())
+
+
+def main():
+    args = get_args()
+    assert Path(args.lexicon).is_file()
+    assert Path(args.transcript).is_file()
+    assert len(args.oov) > 0
+
+    # Only the last pronunciation of a word is kept
+    lexicon = dict(read_lexicon(args.lexicon))
+
+    assert args.oov in lexicon
+
+    oov_token = lexicon[args.oov]
+
+    with open(args.transcript) as f:
+        for line in f:
+            process_line(lexicon=lexicon, line=line, oov_token=oov_token)
+
+
+if __name__ == "__main__":
+    main()

egs/librispeech/ASR/local/prepare_lang.py

@@ -32,7 +32,10 @@ consisting of words and tokens (i.e., phones) and does the following:
         lexicon = k2.Fsa.from_dict(d)
 
 5. Generate L_disambig.pt, in k2 format.
+
+The generated files are saved into `lang_dir`.
 """
+import argparse
 import math
 from collections import defaultdict
 from pathlib import Path
@@ -46,6 +49,19 @@ from icefall.lexicon import read_lexicon, write_lexicon
 Lexicon = List[Tuple[str, List[str]]]
 
 
+def get_args():
+    parser = argparse.ArgumentParser()
+    parser.add_argument(
+        "--lang-dir",
+        type=str,
+        help="""Input and output directory.
+        It should contain a file lexicon.txt
+        """,
+    )
+
+    return parser.parse_args()
+
+
 def write_mapping(filename: str, sym2id: Dict[str, int]) -> None:
     """Write a symbol to ID mapping to a file.
 
@@ -315,8 +331,10 @@ def lexicon_to_fst(
 
 
 def main():
-    out_dir = Path("data/lang_phone")
+    args = get_args()
-    lexicon_filename = out_dir / "lexicon.txt"
+    lang_dir = Path(args.lang_dir)
+
+    lexicon_filename = lang_dir / "lexicon.txt"
     sil_token = "SIL"
     sil_prob = 0.5
 
@@ -344,9 +362,9 @@ def main():
     token2id = generate_id_map(tokens)
     word2id = generate_id_map(words)
 
-    write_mapping(out_dir / "tokens.txt", token2id)
+    write_mapping(lang_dir / "tokens.txt", token2id)
-    write_mapping(out_dir / "words.txt", word2id)
+    write_mapping(lang_dir / "words.txt", word2id)
-    write_lexicon(out_dir / "lexicon_disambig.txt", lexicon_disambig)
+    write_lexicon(lang_dir / "lexicon_disambig.txt", lexicon_disambig)
 
     L = lexicon_to_fst(
         lexicon,
@@ -364,17 +382,8 @@ def main():
         sil_prob=sil_prob,
         need_self_loops=True,
     )
-    torch.save(L.as_dict(), out_dir / "L.pt")
+    torch.save(L.as_dict(), lang_dir / "L.pt")
-    torch.save(L_disambig.as_dict(), out_dir / "L_disambig.pt")
+    torch.save(L_disambig.as_dict(), lang_dir / "L_disambig.pt")
-
-    if False:
-        # Just for debugging, will remove it
-        L.labels_sym = k2.SymbolTable.from_file(out_dir / "tokens.txt")
-        L.aux_labels_sym = k2.SymbolTable.from_file(out_dir / "words.txt")
-        L_disambig.labels_sym = L.labels_sym
-        L_disambig.aux_labels_sym = L.aux_labels_sym
-        L.draw(out_dir / "L.png", title="L")
-        L_disambig.draw(out_dir / "L_disambig.png", title="L_disambig")
 
 
 if __name__ == "__main__":

egs/librispeech/ASR/prepare.sh

@@ -113,14 +113,60 @@ fi
 
 if [ $stage -le 5 ] && [ $stop_stage -ge 5 ]; then
   log "Stage 5: Prepare phone based lang"
-  mkdir -p data/lang_phone
+  lang_dir=data/lang_phone
+  mkdir -p $lang_dir
 
   (echo '!SIL SIL'; echo '<SPOKEN_NOISE> SPN'; echo '<UNK> SPN'; ) |
     cat - $dl_dir/lm/librispeech-lexicon.txt |
-    sort | uniq > data/lang_phone/lexicon.txt
+    sort | uniq > $lang_dir/lexicon.txt
 
-  if [ ! -f data/lang_phone/L_disambig.pt ]; then
+  if [ ! -f $lang_dir/L_disambig.pt ]; then
-    ./local/prepare_lang.py
+    ./local/prepare_lang.py --lang-dir $lang_dir
+  fi
+
+  # Train a bigram P for MMI training
+  if [ ! -f $lang_dir/train.txt ]; then
+    log "Generate data to train phone based bigram P"
+    files=$(
+      find -L "$dl_dir/LibriSpeech/train-clean-100" -name "*.trans.txt"
+      find -L "$dl_dir/LibriSpeech/train-clean-360" -name "*.trans.txt"
+      find -L "$dl_dir/LibriSpeech/train-other-500" -name "*.trans.txt"
+    )
+    for f in ${files[@]}; do
+      cat $f | cut -d " " -f 2-
+    done > $lang_dir/train.txt
+  fi
+
+  if [ ! -f $lang_dir/train_with_sil.txt ]; then
+    ./local/add_silence_to_transcript.py \
+      --transcript $lang_dir/train.txt \
+      --sil-word "!SIL" \
+      --sil-prob 0.5 \
+      --seed 20210823 \
+      > $lang_dir/train_with_sil.txt
+  fi
+
+  if [ ! -f $lang_dir/corpus.txt ]; then
+    ./local/convert_transcript_to_corpus.py \
+      --lexicon $lang_dir/lexicon.txt \
+      --transcript $lang_dir/train_with_sil.txt \
+      --oov "<UNK>" \
+      > $lang_dir/corpus.txt
+  fi
+
+  if [ ! -f $lang_dir/P.arpa ]; then
+    ./shared/make_kn_lm.py \
+      -ngram-order 2 \
+      -text $lang_dir/corpus.txt \
+      -lm $lang_dir/P.arpa
+  fi
+
+  if [ ! -f $lang_dir/P.fst.txt ]; then
+    python3 -m kaldilm \
+      --read-symbol-table="$lang_dir/tokens.txt" \
+      --disambig-symbol='#0' \
+      --max-order=2 \
+      $lang_dir/P.arpa > $lang_dir/P.fst.txt
   fi
 fi
 

icefall/mmi.py

@@ -0,0 +1,222 @@
+from typing import List, Union
+
+import k2
+import torch
+from torch import nn
+
+from icefall.mmi_graph_compiler import MmiTrainingGraphCompiler
+
+
+def _compute_mmi_loss_exact_optimized(
+    dense_fsa_vec: k2.DenseFsaVec,
+    texts: List[str],
+    graph_compiler: MmiTrainingGraphCompiler,
+    den_scale: float = 1.0,
+) -> torch.Tensor:
+    """
+    The function name contains `exact`, which means it uses a version of
+    intersection without pruning.
+
+    `optimized` in the function name means this function is optimized
+    in that it calls k2.intersect_dense only once
+
+    Note:
+      It is faster at the cost of using more memory.
+
+    Args:
+      dense_fsa_vec:
+        It contains the neural network output.
+      texts:
+        The transcript. Each element consists of space(s) separated words.
+      graph_compiler:
+        Used to build num_graphs and den_graphs
+      den_scale:
+        The scale applied to the denominator tot_scores.
+    Returns:
+      Return a scalar loss. It is the sum over utterances in a batch,
+      without normalization.
+    """
+    num_graphs, den_graphs = graph_compiler.compile(texts, replicate_den=False)
+
+    device = num_graphs.device
+
+    num_fsas = num_graphs.shape[0]
+    assert dense_fsa_vec.dim0() == num_fsas
+
+    assert den_graphs.shape[0] == 1
+
+    # The motivation to concatenate num_graphs and den_graphs
+    # is to reduce the number of calls to k2.intersect_dense.
+    num_den_graphs = k2.cat([num_graphs, den_graphs])
+
+    # NOTE: The a_to_b_map in k2.intersect_dense must be sorted
+    # so the following reorders num_den_graphs.
+    #
+    # The following code computes a_to_b_map
+
+    # [0, 1, 2, ... ]
+    num_graphs_indexes = torch.arange(num_fsas, dtype=torch.int32)
+
+    # [num_fsas, num_fsas, num_fsas, ... ]
+    den_graphs_indexes = torch.tensor([num_fsas] * num_fsas, dtype=torch.int32)
+
+    # [0, num_fsas, 1, num_fsas, 2, num_fsas, ... ]
+    num_den_graphs_indexes = (
+        torch.stack([num_graphs_indexes, den_graphs_indexes])
+        .t()
+        .reshape(-1)
+        .to(device)
+    )
+
+    num_den_reordered_graphs = k2.index(num_den_graphs, num_den_graphs_indexes)
+
+    # [[0, 1, 2, ...]]
+    a_to_b_map = torch.arange(num_fsas, dtype=torch.int32).reshape(1, -1)
+
+    # [[0, 1, 2, ...]] -> [0, 0, 1, 1, 2, 2, ... ]
+    a_to_b_map = a_to_b_map.repeat(2, 1).t().reshape(-1).to(device)
+
+    num_den_lats = k2.intersect_dense(
+        num_den_reordered_graphs,
+        dense_fsa_vec,
+        output_beam=8.0,
+        a_to_b_map=a_to_b_map,
+    )
+
+    num_den_tot_scores = num_den_lats.get_tot_scores(
+        log_semiring=True, use_double_scores=True
+    )
+
+    num_tot_scores = num_den_tot_scores[::2]
+    den_tot_scores = num_den_tot_scores[1::2]
+
+    tot_scores = num_tot_scores - den_scale * den_tot_scores
+    loss = -1 * tot_scores.sum()
+    return loss
+
+
+def _compute_mmi_loss_exact_non_optimized(
+    dense_fsa_vec: k2.DenseFsaVec,
+    texts: List[str],
+    graph_compiler: MmiTrainingGraphCompiler,
+    den_scale: float = 1.0,
+) -> torch.Tensor:
+    """
+    See :func:`_compute_mmi_loss_exact_optimized` for the meaning
+    of the arguments.
+
+    It's more readable, though it invokes k2.intersect_dense twice.
+
+    Note:
+      It uses less memory at the cost of speed. It is slower.
+    """
+    num_graphs, den_graphs = graph_compiler.compile(texts, replicate_den=True)
+
+    # TODO: pass output_beam as function argument
+    num_lats = k2.intersect_dense(num_graphs, dense_fsa_vec, output_beam=8.0)
+    den_lats = k2.intersect_dense(den_graphs, dense_fsa_vec, output_beam=8.0)
+
+    num_tot_scores = num_lats.get_tot_scores(
+        log_semiring=True, use_double_scores=True
+    )
+
+    den_tot_scores = den_lats.get_tot_scores(
+        log_semiring=True, use_double_scores=True
+    )
+
+    tot_scores = num_tot_scores - den_scale * den_tot_scores
+
+    loss = -1 * tot_scores.sum()
+    return loss
+
+
+def _compute_mmi_loss_pruned(
+    dense_fsa_vec: k2.DenseFsaVec,
+    texts: List[str],
+    graph_compiler: MmiTrainingGraphCompiler,
+    den_scale: float = 1.0,
+) -> torch.Tensor:
+    """
+    See :func:`_compute_mmi_loss_exact_optimized` for the meaning
+    of the arguments.
+
+    `pruned` means it uses k2.intersect_dense_pruned
+
+    Note:
+      It uses the least amount of memory, but the loss is not exact due
+      to pruning.
+    """
+    num_graphs, den_graphs = graph_compiler.compile(texts, replicate_den=False)
+
+    num_lats = k2.intersect_dense(num_graphs, dense_fsa_vec, output_beam=10.0)
+
+    # the values for search_beam/output_beam/min_active_states/max_active_states
+    # are not tuned. You may want to tune them.
+    den_lats = k2.intersect_dense_pruned(
+        den_graphs,
+        dense_fsa_vec,
+        search_beam=20.0,
+        output_beam=8.0,
+        min_active_states=30,
+        max_active_states=10000,
+    )
+
+    num_tot_scores = num_lats.get_tot_scores(
+        log_semiring=True, use_double_scores=True
+    )
+
+    den_tot_scores = den_lats.get_tot_scores(
+        log_semiring=True, use_double_scores=True
+    )
+
+    tot_scores = num_tot_scores - den_scale * den_tot_scores
+
+    loss = -1 * tot_scores.sum()
+    return loss
+
+
+class LFMMILoss(nn.Module):
+    """
+    Computes Lattice-Free Maximum Mutual Information (LFMMI) loss.
+
+    TODO: more detailed description
+    """
+
+    def __init__(
+        self,
+        graph_compiler: MmiTrainingGraphCompiler,
+        use_pruned_intersect: bool = False,
+        den_scale: float = 1.0,
+    ):
+        super().__init__()
+        self.graph_compiler = graph_compiler
+        self.den_scale = den_scale
+        self.use_pruned_intersect = use_pruned_intersect
+
+    def forward(
+        self,
+        dense_fsa_vec: k2.DenseFsaVec,
+        texts: List[str],
+    ) -> torch.Tensor:
+        """
+        Args:
+          dense_fsa_vec:
+            It contains the neural network output.
+          texts:
+            A list of strings. Each string contains space(s) separated words.
+        Returns:
+          Return a scalar loss. It is the sum over utterances in a batch,
+          without normalization.
+        """
+        if self.use_pruned_intersect:
+            func = _compute_mmi_loss_pruned
+        else:
+            func = _compute_mmi_loss_exact_non_optimized
+            #  func = _compute_mmi_loss_exact_optimized
+
+        return func(
+            dense_fsa_vec=dense_fsa_vec,
+            texts=texts,
+            graph_compiler=self.graph_compiler,
+            den_scale=self.den_scale,
+        )

icefall/mmi_graph_compiler.py

@@ -0,0 +1,184 @@
+from typing import Iterable, List, Tuple, Union
+import logging
+
+import k2
+import torch
+from pathlib import Path
+
+from icefall.lexicon import Lexicon
+
+
+class MmiTrainingGraphCompiler(object):
+    def __init__(
+        self,
+        lang_dir: Path,
+        device: Union[str, torch.device] = "cpu",
+        oov: str = "<UNK>",
+    ):
+        """
+        Args:
+          lang_dir:
+            Path to the lang directory. It is expected to contain the
+            following files::
+
+                - tokens.txt
+                - words.txt
+                - P.fst.txt
+
+            The above files are generated by the script `prepare.sh`. You
+            should have run it before running the training code.
+
+          device:
+            It indicates CPU or CUDA.
+          oov:
+            Out of vocabulary word. When a word in the transcript
+            does not exist in the lexicon, it is replaced with `oov`.
+        """
+        self.lang_dir = Path(lang_dir)
+        self.lexicon = Lexicon(lang_dir)
+        self.device = torch.device(device)
+
+        self.L_inv = self.lexicon.L_inv.to(self.device)
+
+        self.oov_id = self.lexicon.word_table[oov]
+
+        self.build_ctc_topo_P()
+
+    def build_ctc_topo_P(self):
+        """Built ctc_topo_P, the composition result of
+        ctc_topo and P, where P is a pre-trained bigram
+        word piece LM.
+        """
+        # Note: there is no need to save a pre-compiled P and ctc_topo
+        # as it is very fast to generate them.
+        logging.info(f"Loading P from {self.lang_dir/'P.fst.txt'}")
+        with open(self.lang_dir / "P.fst.txt") as f:
+            # P is not an acceptor because there is
+            # a back-off state, whose incoming arcs
+            # have label #0 and aux_label 0 (i.e., <eps>).
+            P = k2.Fsa.from_openfst(f.read(), acceptor=False)
+
+        first_token_disambig_id = self.lexicon.token_table["#0"]
+
+        # P.aux_labels is not needed in later computations, so
+        # remove it here.
+        del P.aux_labels
+        # CAUTION: The following line is crucial.
+        # Arcs entering the back-off state have label equal to #0.
+        # We have to change it to 0 here.
+        P.labels[P.labels >= first_token_disambig_id] = 0
+
+        P = k2.remove_epsilon(P)
+        P = k2.arc_sort(P)
+        P = P.to(self.device)
+        # Add epsilon self-loops to P because we want the
+        # following operation "k2.intersect" to run on GPU.
+        P_with_self_loops = k2.add_epsilon_self_loops(P)
+
+        max_token_id = max(self.lexicon.tokens)
+        logging.info(f"Building ctc_topo. max_token_id: {max_token_id}")
+        ctc_topo = k2.ctc_topo(max_token_id, modified=False, device=self.device)
+
+        ctc_topo_inv = k2.arc_sort(ctc_topo.invert_())
+
+        logging.info("Building ctc_topo_P")
+        ctc_topo_P = k2.intersect(
+            ctc_topo_inv, P_with_self_loops, treat_epsilons_specially=False
+        ).invert()
+
+        self.ctc_topo_P = k2.arc_sort(ctc_topo_P)
+
+    def compile(
+        self, texts: Iterable[str], replicate_den: bool = True
+    ) -> Tuple[k2.Fsa, k2.Fsa]:
+        """Create numerator and denominator graphs from transcripts
+        and the bigram phone LM.
+
+        Args:
+          texts:
+            A list of transcripts. Within a transcript, words are
+            separated by spaces.
+          replicate_den:
+            If True, the returned den_graph is replicated to match the number
+            of FSAs in the returned num_graph; if False, the returned den_graph
+            contains only a single FSA
+        Returns:
+          A tuple (num_graph, den_graph), where
+
+            - `num_graph` is the numerator graph. It is an FsaVec with
+              shape `(len(texts), None, None)`.
+
+            - `den_graph` is the denominator graph. It is an FsaVec
+              with the same shape of the `num_graph` if replicate_den is
+              True; otherwise, it is an FsaVec containing only a single FSA.
+        """
+        transcript_fsa = self.build_transcript_fsa(texts)
+
+        # remove word IDs from transcript_fsa since it is not needed
+        del transcript_fsa.aux_labels
+
+        transcript_fsa_with_self_loops = k2.remove_epsilon_and_add_self_loops(
+            transcript_fsa
+        )
+
+        transcript_fsa_with_self_loops = k2.arc_sort(
+            transcript_fsa_with_self_loops
+        )
+
+        num = k2.compose(
+            self.ctc_topo_P,
+            transcript_fsa_with_self_loops,
+            treat_epsilons_specially=False,
+        )
+        num = k2.arc_sort(num)
+
+        ctc_topo_P_vec = k2.create_fsa_vec([self.ctc_topo_P])
+        if replicate_den:
+            indexes = torch.zeros(
+                len(texts), dtype=torch.int32, device=self.device
+            )
+            den = k2.index_fsa(ctc_topo_P_vec, indexes)
+        else:
+            den = ctc_topo_P_vec
+
+        return num, den
+
+    def build_transcript_fsa(self, texts: List[str]) -> k2.Fsa:
+        """Convert transcripts to an FsaVec with the help of a lexicon
+        and word symbol table.
+
+        Args:
+          texts:
+            Each element is a transcript containing words separated by space(s).
+            For instance, it may be 'HELLO icefall', which contains
+            two words.
+
+        Returns:
+          Return an FST (FsaVec) corresponding to the transcript.
+          Its `labels` is token IDs and `aux_labels` is word IDs.
+        """
+        word_ids_list = []
+        for text in texts:
+            word_ids = []
+            for word in text.split(" "):
+                if word in self.lexicon.word_table:
+                    word_ids.append(self.lexicon.word_table[word])
+                else:
+                    word_ids.append(self.oov_id)
+            word_ids_list.append(word_ids)
+
+        fsa = k2.linear_fsa(word_ids_list, self.device)
+        fsa = k2.add_epsilon_self_loops(fsa)
+
+        # The reason to use `invert_()` at the end is as follows:
+        #
+        # (1) The `labels` of L_inv is word IDs and `aux_labels` is token IDs
+        # (2) `fsa.labels` is word IDs
+        # (3) after intersection, the `labels` is still word IDs
+        # (4) after `invert_()`, the `labels` is token IDs
+        #     and `aux_labels` is word IDs
+        transcript_fsa = k2.intersect(
+            self.L_inv, fsa, treat_epsilons_specially=False
+        ).invert_()
+        transcript_fsa = k2.arc_sort(transcript_fsa)
+        return transcript_fsa

icefall/shared/make_kn_lm.py

@@ -0,0 +1,377 @@
+#!/usr/bin/env python3
+
+# Copyright 2016  Johns Hopkins University (Author: Daniel Povey)
+#           2018  Ruizhe Huang
+# Apache 2.0.
+
+# This is an implementation of computing Kneser-Ney smoothed language model
+# in the same way as srilm. This is a back-off, unmodified version of
+# Kneser-Ney smoothing, which produces the same results as the following
+# command (as an example) of srilm:
+#
+# $ ngram-count -order 4 -kn-modify-counts-at-end -ukndiscount -gt1min 0 -gt2min 0 -gt3min 0 -gt4min 0 \
+# -text corpus.txt -lm lm.arpa
+#
+# The data structure is based on: kaldi/egs/wsj/s5/utils/lang/make_phone_lm.py
+# The smoothing algorithm is based on: http://www.speech.sri.com/projects/srilm/manpages/ngram-discount.7.html
+
+import sys
+import os
+import re
+import io
+import math
+import argparse
+from collections import Counter, defaultdict
+
+
+parser = argparse.ArgumentParser(description="""
+    Generate kneser-ney language model as arpa format. By default,
+    it will read the corpus from standard input, and output to standard output.
+    """)
+parser.add_argument("-ngram-order", type=int, default=4, choices=[2, 3, 4, 5, 6, 7], help="Order of n-gram")
+parser.add_argument("-text", type=str, default=None, help="Path to the corpus file")
+parser.add_argument("-lm", type=str, default=None, help="Path to output arpa file for language models")
+parser.add_argument("-verbose", type=int, default=0, choices=[0, 1, 2, 3, 4, 5], help="Verbose level")
+args = parser.parse_args()
+
+default_encoding = "latin-1"  # For encoding-agnostic scripts, we assume byte stream as input.
+                              # Need to be very careful about the use of strip() and split()
+                              # in this case, because there is a latin-1 whitespace character
+                              # (nbsp) which is part of the unicode encoding range.
+                              # Ref: kaldi/egs/wsj/s5/utils/lang/bpe/prepend_words.py @ 69cd717
+strip_chars = " \t\r\n"
+whitespace = re.compile("[ \t]+")
+
+
+class CountsForHistory:
+    # This class (which is more like a struct) stores the counts seen in a
+    # particular history-state.  It is used inside class NgramCounts.
+    # It really does the job of a dict from int to float, but it also
+    # keeps track of the total count.
+    def __init__(self):
+        # The 'lambda: defaultdict(float)' is an anonymous function taking no
+        # arguments that returns a new defaultdict(float).
+        self.word_to_count = defaultdict(int)
+        self.word_to_context = defaultdict(set)  # using a set to count the number of unique contexts
+        self.word_to_f = dict()  # discounted probability
+        self.word_to_bow = dict()  # back-off weight
+        self.total_count = 0
+
+    def words(self):
+        return self.word_to_count.keys()
+
+    def __str__(self):
+        # e.g. returns ' total=12: 3->4, 4->6, -1->2'
+        return ' total={0}: {1}'.format(
+            str(self.total_count),
+            ', '.join(['{0} -> {1}'.format(word, count)
+                      for word, count in self.word_to_count.items()]))
+
+    def add_count(self, predicted_word, context_word, count):
+        assert count >= 0
+
+        self.total_count += count
+        self.word_to_count[predicted_word] += count
+        if context_word is not None:
+            self.word_to_context[predicted_word].add(context_word)
+
+
+class NgramCounts:
+    # A note on data-structure.  Firstly, all words are represented as
+    # integers.  We store n-gram counts as an array, indexed by (history-length
+    # == n-gram order minus one) (note: python calls arrays "lists") of dicts
+    # from histories to counts, where histories are arrays of integers and
+    # "counts" are dicts from integer to float.  For instance, when
+    # accumulating the 4-gram count for the '8' in the sequence '5 6 7 8', we'd
+    # do as follows: self.counts[3][[5,6,7]][8] += 1.0 where the [3] indexes an
+    # array, the [[5,6,7]] indexes a dict, and the [8] indexes a dict.
+    def __init__(self, ngram_order, bos_symbol='<s>', eos_symbol='</s>'):
+        assert ngram_order >= 2
+
+        self.ngram_order = ngram_order
+        self.bos_symbol = bos_symbol
+        self.eos_symbol = eos_symbol
+
+        self.counts = []
+        for n in range(ngram_order):
+            self.counts.append(defaultdict(lambda: CountsForHistory()))
+
+        self.d = []  # list of discounting factor for each order of ngram
+
+    # adds a raw count (called while processing input data).
+    # Suppose we see the sequence '6 7 8 9' and ngram_order=4, 'history'
+    # would be (6,7,8) and 'predicted_word' would be 9; 'count' would be
+    # 1.
+    def add_count(self, history, predicted_word, context_word, count):
+        self.counts[len(history)][history].add_count(predicted_word, context_word, count)
+
+    # 'line' is a string containing a sequence of integer word-ids.
+    # This function adds the un-smoothed counts from this line of text.
+    def add_raw_counts_from_line(self, line):
+        if line == '':
+            words = [self.bos_symbol, self.eos_symbol]
+        else:
+            words = [self.bos_symbol] + whitespace.split(line) + [self.eos_symbol]
+
+        for i in range(len(words)):
+            for n in range(1, self.ngram_order+1):
+                if i + n > len(words):
+                    break
+                ngram = words[i: i + n]
+                predicted_word = ngram[-1]
+                history = tuple(ngram[: -1])
+                if i == 0 or n == self.ngram_order:
+                    context_word = None
+                else:
+                    context_word = words[i-1]
+
+                self.add_count(history, predicted_word, context_word, 1)
+
+    def add_raw_counts_from_standard_input(self):
+        lines_processed = 0
+        infile = io.TextIOWrapper(sys.stdin.buffer, encoding=default_encoding)  # byte stream as input
+        for line in infile:
+            line = line.strip(strip_chars)
+            self.add_raw_counts_from_line(line)
+            lines_processed += 1
+        if lines_processed == 0 or args.verbose > 0:
+            print("make_phone_lm.py: processed {0} lines of input".format(lines_processed), file=sys.stderr)
+
+    def add_raw_counts_from_file(self, filename):
+        lines_processed = 0
+        with open(filename, encoding=default_encoding) as fp:
+            for line in fp:
+                line = line.strip(strip_chars)
+                self.add_raw_counts_from_line(line)
+                lines_processed += 1
+        if lines_processed == 0 or args.verbose > 0:
+            print("make_phone_lm.py: processed {0} lines of input".format(lines_processed), file=sys.stderr)
+
+    def cal_discounting_constants(self):
+        # For each order N of N-grams, we calculate discounting constant D_N = n1_N / (n1_N + 2 * n2_N),
+        # where n1_N is the number of unique N-grams with count = 1 (counts-of-counts).
+        # This constant is used similarly to absolute discounting.
+        # Return value: d is a list of floats, where d[N+1] = D_N
+
+        self.d = [0]  # for the lowest order, i.e., 1-gram, we do not need to discount, thus the constant is 0
+                      # This is a special case: as we currently assumed having seen all vocabularies in the dictionary,
+                      # but perhaps this is not the case for some other scenarios.
+        for n in range(1, self.ngram_order):
+            this_order_counts = self.counts[n]
+            n1 = 0
+            n2 = 0
+            for hist, counts_for_hist in this_order_counts.items():
+                stat = Counter(counts_for_hist.word_to_count.values())
+                n1 += stat[1]
+                n2 += stat[2]
+            assert n1 + 2 * n2 > 0
+            self.d.append(n1 * 1.0 / (n1 + 2 * n2))
+
+    def cal_f(self):
+        # f(a_z) is a probability distribution of word sequence a_z.
+        # Typically f(a_z) is discounted to be less than the ML estimate so we have
+        # some leftover probability for the z words unseen in the context (a_).
+        #
+        # f(a_z) = (c(a_z) - D0) / c(a_)    ;; for highest order N-grams
+        # f(_z)  = (n(*_z) - D1) / n(*_*)	;; for lower order N-grams
+
+        # highest order N-grams
+        n = self.ngram_order - 1
+        this_order_counts = self.counts[n]
+        for hist, counts_for_hist in this_order_counts.items():
+            for w, c in counts_for_hist.word_to_count.items():
+                counts_for_hist.word_to_f[w] = max((c - self.d[n]), 0) * 1.0 / counts_for_hist.total_count
+
+        # lower order N-grams
+        for n in range(0, self.ngram_order - 1):
+            this_order_counts = self.counts[n]
+            for hist, counts_for_hist in this_order_counts.items():
+
+                n_star_star = 0
+                for w in counts_for_hist.word_to_count.keys():
+                    n_star_star += len(counts_for_hist.word_to_context[w])
+
+                if n_star_star != 0:
+                    for w in counts_for_hist.word_to_count.keys():
+                        n_star_z = len(counts_for_hist.word_to_context[w])
+                        counts_for_hist.word_to_f[w] = max((n_star_z - self.d[n]), 0) * 1.0 / n_star_star
+                else:  # patterns begin with <s>, they do not have "modified count", so use raw count instead
+                    for w in counts_for_hist.word_to_count.keys():
+                        n_star_z = counts_for_hist.word_to_count[w]
+                        counts_for_hist.word_to_f[w] = max((n_star_z - self.d[n]), 0) * 1.0 / counts_for_hist.total_count
+
+    def cal_bow(self):
+        # Backoff weights are only necessary for ngrams which form a prefix of a longer ngram.
+        # Thus, two sorts of ngrams do not have a bow:
+        # 1) highest order ngram
+        # 2) ngrams ending in </s>
+        #
+        # bow(a_) = (1 - Sum_Z1 f(a_z)) / (1 - Sum_Z1 f(_z))
+        # Note that Z1 is the set of all words with c(a_z) > 0
+
+        # highest order N-grams
+        n = self.ngram_order - 1
+        this_order_counts = self.counts[n]
+        for hist, counts_for_hist in this_order_counts.items():
+            for w in counts_for_hist.word_to_count.keys():
+                counts_for_hist.word_to_bow[w] = None
+
+        # lower order N-grams
+        for n in range(0, self.ngram_order - 1):
+            this_order_counts = self.counts[n]
+            for hist, counts_for_hist in this_order_counts.items():
+                for w in counts_for_hist.word_to_count.keys():
+                    if w == self.eos_symbol:
+                        counts_for_hist.word_to_bow[w] = None
+                    else:
+                        a_ = hist + (w,)
+
+                        assert len(a_) < self.ngram_order
+                        assert a_ in self.counts[len(a_)].keys()
+
+                        a_counts_for_hist = self.counts[len(a_)][a_]
+
+                        sum_z1_f_a_z = 0
+                        for u in a_counts_for_hist.word_to_count.keys():
+                            sum_z1_f_a_z += a_counts_for_hist.word_to_f[u]
+
+                        sum_z1_f_z = 0
+                        _ = a_[1:]
+                        _counts_for_hist = self.counts[len(_)][_]
+                        for u in a_counts_for_hist.word_to_count.keys():  # Should be careful here: what is Z1
+                            sum_z1_f_z += _counts_for_hist.word_to_f[u]
+
+                        counts_for_hist.word_to_bow[w] = (1.0 - sum_z1_f_a_z) / (1.0 - sum_z1_f_z)
+
+    def print_raw_counts(self, info_string):
+        # these are useful for debug.
+        print(info_string)
+        res = []
+        for this_order_counts in self.counts:
+            for hist, counts_for_hist in this_order_counts.items():
+                for w in counts_for_hist.word_to_count.keys():
+                    ngram = " ".join(hist) + " " + w
+                    ngram = ngram.strip(strip_chars)
+
+                    res.append("{0}\t{1}".format(ngram, counts_for_hist.word_to_count[w]))
+        res.sort(reverse=True)
+        for r in res:
+            print(r)
+
+    def print_modified_counts(self, info_string):
+        # these are useful for debug.
+        print(info_string)
+        res = []
+        for this_order_counts in self.counts:
+            for hist, counts_for_hist in this_order_counts.items():
+                for w in counts_for_hist.word_to_count.keys():
+                    ngram = " ".join(hist) + " " + w
+                    ngram = ngram.strip(strip_chars)
+
+                    modified_count = len(counts_for_hist.word_to_context[w])
+                    raw_count = counts_for_hist.word_to_count[w]
+
+                    if modified_count == 0:
+                        res.append("{0}\t{1}".format(ngram, raw_count))
+                    else:
+                        res.append("{0}\t{1}".format(ngram, modified_count))
+        res.sort(reverse=True)
+        for r in res:
+            print(r)
+
+    def print_f(self, info_string):
+        # these are useful for debug.
+        print(info_string)
+        res = []
+        for this_order_counts in self.counts:
+            for hist, counts_for_hist in this_order_counts.items():
+                for w in counts_for_hist.word_to_count.keys():
+                    ngram = " ".join(hist) + " " + w
+                    ngram = ngram.strip(strip_chars)
+
+                    f = counts_for_hist.word_to_f[w]
+                    if f == 0:  # f(<s>) is always 0
+                        f = 1e-99
+
+                    res.append("{0}\t{1}".format(ngram, math.log(f, 10)))
+        res.sort(reverse=True)
+        for r in res:
+            print(r)
+
+    def print_f_and_bow(self, info_string):
+        # these are useful for debug.
+        print(info_string)
+        res = []
+        for this_order_counts in self.counts:
+            for hist, counts_for_hist in this_order_counts.items():
+                for w in counts_for_hist.word_to_count.keys():
+                    ngram = " ".join(hist) + " " + w
+                    ngram = ngram.strip(strip_chars)
+
+                    f = counts_for_hist.word_to_f[w]
+                    if f == 0:  # f(<s>) is always 0
+                        f = 1e-99
+
+                    bow = counts_for_hist.word_to_bow[w]
+                    if bow is None:
+                        res.append("{1}\t{0}".format(ngram, math.log(f, 10)))
+                    else:
+                        res.append("{1}\t{0}\t{2}".format(ngram, math.log(f, 10), math.log(bow, 10)))
+        res.sort(reverse=True)
+        for r in res:
+            print(r)
+
+    def print_as_arpa(self, fout=io.TextIOWrapper(sys.stdout.buffer, encoding='latin-1')):
+        # print as ARPA format.
+
+        print('\\data\\', file=fout)
+        for hist_len in range(self.ngram_order):
+            # print the number of n-grams.
+            print('ngram {0}={1}'.format(
+                hist_len + 1,
+                sum([len(counts_for_hist.word_to_f) for counts_for_hist in self.counts[hist_len].values()])),
+                file=fout
+            )
+
+        print('', file=fout)
+
+        for hist_len in range(self.ngram_order):
+            print('\\{0}-grams:'.format(hist_len + 1), file=fout)
+
+            this_order_counts = self.counts[hist_len]
+            for hist, counts_for_hist in this_order_counts.items():
+                for word in counts_for_hist.word_to_count.keys():
+                    ngram = hist + (word,)
+                    prob = counts_for_hist.word_to_f[word]
+                    bow = counts_for_hist.word_to_bow[word]
+
+                    if prob == 0:  # f(<s>) is always 0
+                        prob = 1e-99
+
+                    line = '{0}\t{1}'.format('%.7f' % math.log10(prob), ' '.join(ngram))
+                    if bow is not None:
+                        line += '\t{0}'.format('%.7f' % math.log10(bow))
+                    print(line, file=fout)
+            print('', file=fout)
+        print('\\end\\', file=fout)
+
+
+if __name__ == "__main__":
+
+    ngram_counts = NgramCounts(args.ngram_order)
+
+    if args.text is None:
+        ngram_counts.add_raw_counts_from_standard_input()
+    else:
+        assert os.path.isfile(args.text)
+        ngram_counts.add_raw_counts_from_file(args.text)
+
+    ngram_counts.cal_discounting_constants()
+    ngram_counts.cal_f()
+    ngram_counts.cal_bow()
+
+    if args.lm is None:
+        ngram_counts.print_as_arpa()
+    else:
+        with open(args.lm, 'w', encoding=default_encoding) as f:
+            ngram_counts.print_as_arpa(fout=f)