Apply layer bypass during warmup in a new way, including 2s and 4s of layers.

egs/librispeech/ASR/pruned_transducer_stateless7/conformer.py

@@ -49,6 +49,8 @@ class Conformer(EncoderInterface):
         layer_dropout (float): layer-dropout rate.
         cnn_module_kernel (int): Kernel size of convolution module
         vgg_frontend (bool): whether to use vgg frontend.
+        warmup (float): number of batches to warm up over (gradually skip
+                    layer bypass)
     """
 
     def __init__(
@@ -63,6 +65,7 @@ class Conformer(EncoderInterface):
         num_encoder_layers: Tuple[int] = (12, 12),
         dropout: float = 0.1,
         cnn_module_kernel: Tuple[int] = (31, 31),
+        warmup_batches: float = 3000,
     ) -> None:
         super(Conformer, self).__init__()
 
@@ -98,6 +101,7 @@ class Conformer(EncoderInterface):
             encoder_layer1,
             num_encoder_layers[0],
             dropout,
+            warmup_batches,
         )
         encoder_layer2 = ConformerEncoderLayer(
             d_model[1],
@@ -111,6 +115,7 @@ class Conformer(EncoderInterface):
                 encoder_layer2,
                 num_encoder_layers[1],
                 dropout,
+                warmup_batches,
             ),
             input_dim=d_model[0],
             output_dim=d_model[1],
@@ -365,10 +370,16 @@ class ConformerEncoder(nn.Module):
             self,
             encoder_layer: nn.Module,
             num_layers: int,
-        dropout: float
+            dropout: float,
+            warmup_batches: float,
     ) -> None:
         super().__init__()
 
+        # keep track of how many times forward() has been called, for purposes of
+        # 'warmup'
+        self.register_buffer('count', torch.tensor(0, dtype=torch.int64))
+        self.warmup_batches = warmup_batches
+
         self.encoder_pos = RelPositionalEncoding(encoder_layer.d_model,
                                                  dropout)
 
@@ -381,6 +392,18 @@ class ConformerEncoder(nn.Module):
         num_channels = encoder_layer.norm_final.num_channels
 
 
+
+    def get_warmup_value(self) -> float:
+        """
+        Returns a value that is 0 at the start of training and approaches 1.0 after a number of
+        'warmup' batches, specified in the constructor.
+        """
+        batch = self.count.item()
+        if self.training:
+            self.count += 1
+        return min(1.0, batch / self.warmup_batches)
+
+
     def forward(
         self,
         src: Tensor,
@@ -414,30 +437,46 @@ class ConformerEncoder(nn.Module):
 
         output = output * feature_mask
 
-        num_layers = len(self.layers)
-        indexes = list(range(num_layers))
-        if self.training:
-            num_to_swap = int((num_layers - 1) * 0.1)
-            for i in range(num_to_swap):
-                j = random.randint(0, num_layers - 2)
-                a,b = indexes[j], indexes[j+1]
-                indexes[j] = b
-                indexes[j+1] = a
+        outputs = [ output ]
 
+        # warmup starts at 0 at the beginning of training, reaches 1 at a few
+        # thousand minibatches, and then stays there.
+        warmup = self.get_warmup_value()
 
-        for i in indexes:
-            mod = self.layers[i]
+        def apply_bypass(prev_output: Tensor, output: Tensor,
+                         warmup: float,
+                         min_output_scale: float = 0.1,
+                         max_output_scale: float = 1.0):
+            output_scale = max(warmup * max_output_scale,
+                               min_output_scale)
+            if output_scale == 1.0:
+                return output
+            else:
+                return output_scale * output + (1.0 - output_scale) * prev_output
+
+        for i, mod in enumerate(self.layers):
             output, attn_scores = mod(
-                output,
+                outputs[-1],
                 pos_emb,
                 attn_scores,
                 src_mask=mask,
                 src_key_padding_mask=src_key_padding_mask,
             )
-
+            # bypass this layer; the scale on `output` reaches a maximum of 0.5 which
+            # empirically seemed slightly better than 1.
+            output = apply_bypass(outputs[-1], output,
+                                  warmup, 0.1, 0.5)
+            # also apply bypass to twos and fours of layers.
+            if i > 0 and i % 2 == 0:
+                output = apply_bypass(outputs[-2], output,
+                                      warmup, 0.25, 1.0)
+            if i > 0 and i % 4 == 0:
+                output = apply_bypass(outputs[-4], output,
+                                      warmup, 0.25, 1.0)
             output = output * feature_mask
+            outputs.append(output)
 
-        return output
+        return outputs[-1]
 
 
 class DownsampledConformerEncoder(nn.Module):