Add librispeech prefix-beam-search

egs/librispeech/ASR/zipformer/ctc_decode.py

@@ -123,6 +123,10 @@ from asr_datamodule import LibriSpeechAsrDataModule
 from lhotse import set_caching_enabled
 from train import add_model_arguments, get_model, get_params
 
+from icefall.context_graph import ContextGraph, ContextState
+from icefall.ngram_lm import NgramLm, NgramLmStateCost
+from icefall.lm_wrapper import LmScorer
+
 from icefall.checkpoint import (
     average_checkpoints,
     average_checkpoints_with_averaged_model,
@@ -131,6 +135,9 @@ from icefall.checkpoint import (
 )
 from icefall.decode import (
     ctc_greedy_search,
+    ctc_prefix_beam_search,
+    ctc_prefix_beam_search_attention_decoder_rescoring,
+    ctc_prefix_beam_search_shallow_fussion,
     get_lattice,
     nbest_decoding,
     nbest_oracle,
@@ -280,6 +287,23 @@ def get_parser():
         """,
     )
 
+    parser.add_argument(
+        "--lm-type",
+        type=str,
+        default="rnn",
+        help="Type of NN lm",
+        choices=["rnn", "transformer"],
+    )
+
+    parser.add_argument(
+        "--lm-scale",
+        type=float,
+        default=0.3,
+        help="""The scale of the neural network LM
+        Used only when `--use-shallow-fusion` is set to True.
+        """,
+    )
+
     parser.add_argument(
         "--hlg-scale",
         type=float,
@@ -301,7 +325,7 @@ def get_parser():
         "--skip-scoring",
         type=str2bool,
         default=False,
-        help="""Skip scoring, but still save the ASR output (for eval sets)."""
+        help="""Skip scoring, but still save the ASR output (for eval sets).""",
     )
 
     add_model_arguments(parser)
@@ -314,8 +338,9 @@ def get_decoding_params() -> AttributeDict:
     params = AttributeDict(
         {
             "frame_shift_ms": 10,
-            "search_beam": 20,
+            "search_beam": 20,  # for k2 fsa composition
-            "output_beam": 8,
+            "output_beam": 8,  # for k2 fsa composition
+            "beam": 4,  # for prefix-beam-search
             "min_active_states": 30,
             "max_active_states": 10000,
             "use_double_scores": True,
@@ -333,6 +358,7 @@ def decode_one_batch(
     batch: dict,
     word_table: k2.SymbolTable,
     G: Optional[k2.Fsa] = None,
+    LM: Optional[LmScorer] = None,
 ) -> Dict[str, List[List[str]]]:
     """Decode one batch and return the result in a dict. The dict has the
     following format:
@@ -377,10 +403,7 @@ def decode_one_batch(
       Return the decoding result. See above description for the format of
       the returned dict. Note: If it decodes to nothing, then return None.
     """
-    if HLG is not None:
+    device = params.device
-        device = HLG.device
-    else:
-        device = H.device
     feature = batch["inputs"]
     assert feature.ndim == 3
     feature = feature.to(device)
@@ -411,6 +434,48 @@ def decode_one_batch(
         key = "ctc-greedy-search"
         return {key: hyps}
 
+    if params.decoding_method == "prefix-beam-search":
+        token_ids = ctc_prefix_beam_search(
+            ctc_output=ctc_output, encoder_out_lens=encoder_out_lens
+        )
+        # hyps is a list of str, e.g., ['xxx yyy zzz', ...]
+        hyps = bpe_model.decode(token_ids)
+
+        # hyps is a list of list of str, e.g., [['xxx', 'yyy', 'zzz'], ... ]
+        hyps = [s.split() for s in hyps]
+        key = "prefix-beam-search"
+        return {key: hyps}
+
+    if params.decoding_method == "ctc-prefix-beam-search-attention-decoder-rescoring":
+        best_path_dict = ctc_prefix_beam_search_attention_decoder_rescoring(
+            ctc_output=ctc_output,
+            attention_decoder=model.attention_decoder,
+            encoder_out=encoder_out,
+            encoder_out_lens=encoder_out_lens,
+        )
+        ans = dict()
+        for a_scale_str, token_ids in best_path_dict.items():
+            # hyps is a list of str, e.g., ['xxx yyy zzz', ...]
+            hyps = bpe_model.decode(token_ids)
+            # hyps is a list of list of str, e.g., [['xxx', 'yyy', 'zzz'], ... ]
+            hyps = [s.split() for s in hyps]
+            ans[a_scale_str] = hyps
+        return ans
+
+    if params.decoding_method == "ctc-prefix-beam-search-shallow-fussion":
+        token_ids = ctc_prefix_beam_search_shallow_fussion(
+            ctc_output=ctc_output,
+            encoder_out_lens=encoder_out_lens,
+            LM=LM,
+        )
+        # hyps is a list of str, e.g., ['xxx yyy zzz', ...]
+        hyps = bpe_model.decode(token_ids)
+
+        # hyps is a list of list of str, e.g., [['xxx', 'yyy', 'zzz'], ... ]
+        hyps = [s.split() for s in hyps]
+        key = "prefix-beam-search-shallow-fussion"
+        return {key: hyps}
+
     supervision_segments = torch.stack(
         (
             supervisions["sequence_idx"],
@@ -584,6 +649,7 @@ def decode_dataset(
     bpe_model: Optional[spm.SentencePieceProcessor],
     word_table: k2.SymbolTable,
     G: Optional[k2.Fsa] = None,
+    LM: Optional[LmScorer] = None,
 ) -> Dict[str, List[Tuple[str, List[str], List[str]]]]:
     """Decode dataset.
 
@@ -634,6 +700,7 @@ def decode_dataset(
             batch=batch,
             word_table=word_table,
             G=G,
+            LM=LM,
         )
 
         for name, hyps in hyps_dict.items():
@@ -664,9 +731,7 @@ def save_asr_output(
     """
     for key, results in results_dict.items():
 
-        recogs_filename = (
+        recogs_filename = params.res_dir / f"recogs-{test_set_name}-{params.suffix}.txt"
-            params.res_dir / f"recogs-{test_set_name}-{params.suffix}.txt"
-        )
 
         results = sorted(results)
         store_transcripts(filename=recogs_filename, texts=results)
@@ -680,7 +745,8 @@ def save_wer_results(
     results_dict: Dict[str, List[Tuple[str, List[str], List[str]]]],
 ):
     if params.decoding_method in (
-        "attention-decoder-rescoring-with-ngram", "whole-lattice-rescoring"
+        "attention-decoder-rescoring-with-ngram",
+        "whole-lattice-rescoring",
     ):
         # Set it to False since there are too many logs.
         enable_log = False
@@ -721,6 +787,7 @@ def save_wer_results(
 def main():
     parser = get_parser()
     LibriSpeechAsrDataModule.add_arguments(parser)
+    LmScorer.add_arguments(parser)
     args = parser.parse_args()
     args.exp_dir = Path(args.exp_dir)
     args.lang_dir = Path(args.lang_dir)
@@ -735,8 +802,11 @@ def main():
     set_caching_enabled(True)  # lhotse
 
     assert params.decoding_method in (
-        "ctc-greedy-search",
         "ctc-decoding",
+        "ctc-greedy-search",
+        "prefix-beam-search",
+        "ctc-prefix-beam-search-attention-decoder-rescoring",
+        "ctc-prefix-beam-search-shallow-fussion",
         "1best",
         "nbest",
         "nbest-rescoring",
@@ -762,6 +832,11 @@ def main():
         params.suffix += f"_chunk-{params.chunk_size}"
         params.suffix += f"_left-context-{params.left_context_frames}"
 
+    if "prefix-beam-search" in params.decoding_method:
+        params.suffix += f"_beam-{params.beam}"
+        if params.decoding_method == "ctc-prefix-beam-search-shallow-fussion":
+            params.suffix += f"_lm-scale-{params.lm_scale}"
+
     if params.use_averaged_model:
         params.suffix += "_use-averaged-model"
 
@@ -772,6 +847,8 @@ def main():
     if torch.cuda.is_available():
         device = torch.device("cuda", 0)
 
+    params.device = device
+
     logging.info(f"Device: {device}")
     logging.info(params)
 
@@ -786,9 +863,19 @@ def main():
     params.sos_id = 1
 
     if params.decoding_method in [
-        "ctc-greedy-search", "ctc-decoding", "attention-decoder-rescoring-no-ngram"
+        "ctc-greedy-search",
+        "ctc-decoding",
+        "attention-decoder-rescoring-no-ngram",
+        "prefix-beam-search",
+        "ctc-prefix-beam-search-attention-decoder-rescoring",
+        "ctc-prefix-beam-search-shallow-fussion",
     ]:
         HLG = None
+        H = None
+        if params.decoding_method in [
+            "ctc-decoding",
+            "attention-decoder-rescoring-no-ngram",
+        ]:
             H = k2.ctc_topo(
                 max_token=max_token_id,
                 modified=False,
@@ -844,7 +931,8 @@ def main():
             G = k2.Fsa.from_dict(d)
 
         if params.decoding_method in [
-            "whole-lattice-rescoring", "attention-decoder-rescoring-with-ngram"
+            "whole-lattice-rescoring",
+            "attention-decoder-rescoring-with-ngram",
         ]:
             # Add epsilon self-loops to G as we will compose
             # it with the whole lattice later
@@ -858,6 +946,19 @@ def main():
     else:
         G = None
 
+    # only load the neural network LM if required
+    if params.decoding_method == "ctc-prefix-beam-search-shallow-fussion":
+        LM = LmScorer(
+            lm_type=params.lm_type,
+            params=params,
+            device=device,
+            lm_scale=params.lm_scale,
+        )
+        LM.to(device)
+        LM.eval()
+    else:
+        LM = None
+
     logging.info("About to create model")
     model = get_model(params)
 
@@ -967,6 +1068,7 @@ def main():
             bpe_model=bpe_model,
             word_table=lexicon.word_table,
             G=G,
+            LM=LM,
         )
 
         save_asr_output(

icefall/decode.py

@@ -1511,30 +1511,43 @@ def ctc_greedy_search(
 class Hypothesis:
     # The predicted tokens so far.
     # Newly predicted tokens are appended to `ys`.
-    ys: List[int]
+    ys: List[int] = field(default_factory=list)
 
     # The log prob of ys.
     # It contains only one entry.
-    log_prob_blank: torch.Tensor
+    log_prob_blank: torch.Tensor = torch.zeros(1, dtype=torch.float32)
 
-    log_prob_non_blank: torch.Tensor
+    log_prob_non_blank: torch.Tensor = torch.tensor(
+        [float("-inf")], dtype=torch.float32
+    )
 
     # timestamp[i] is the frame index after subsampling
     # on which ys[i] is decoded
     timestamp: List[int] = field(default_factory=list)
 
-    # the lm score for next token given the current ys
+    # The lm score of ys
-    lm_score: Optional[torch.Tensor] = None
+    # It contains only one entry
+    lm_score: torch.Tensor = torch.zeros(1, dtype=torch.float32)
+
+    # the lm log_probs for next token given the history ys
+    lm_log_probs: Optional[torch.Tensor] = None
 
     # the RNNLM states (h and c in LSTM)
     state: Optional[Tuple[torch.Tensor, torch.Tensor]] = None
 
     # N-gram LM state
-    state_cost: Optional[NgramLmStateCost] = None
+    LODR_state: Optional[NgramLmStateCost] = None
+
+    # N-gram LM state
+    Ngram_state: Optional[NgramLmStateCost] = None
 
     # Context graph state
     context_state: Optional[ContextState] = None
 
+    @property
+    def tot_score(self) -> torch.Tensor:
+        return self.log_prob + self.lm_score
+
     @property
     def log_prob(self) -> torch.Tensor:
         return torch.logaddexp(self.log_prob_non_blank, self.log_prob_blank)
@@ -1544,6 +1557,20 @@ class Hypothesis:
         """Return a tuple representation of self.ys"""
         return tuple(self.ys)
 
+    def clone(self) -> "Hypothesis":
+        return Hypothesis(
+            ys=self.ys,
+            log_prob_blank=self.log_prob_blank,
+            log_prob_non_blank=self.log_prob_non_blank,
+            timestamp=self.timestamp,
+            lm_log_probs=self.lm_log_probs,
+            lm_score=self.lm_score,
+            state=self.state,
+            LODR_state=self.LODR_state,
+            Ngram_state=self.Ngram_state,
+            context_state=self.context_state,
+        )
+
 
 class HypothesisList(object):
     def __init__(self, data: Optional[Dict[tuple, Hypothesis]] = None) -> None:
@@ -1597,9 +1624,9 @@ class HypothesisList(object):
           Return the hypothesis that has the largest `log_prob`.
         """
         if length_norm:
-            return max(self._data.values(), key=lambda hyp: hyp.log_prob / len(hyp.ys))
+            return max(self._data.values(), key=lambda hyp: hyp.tot_score / len(hyp.ys))
         else:
-            return max(self._data.values(), key=lambda hyp: hyp.log_prob)
+            return max(self._data.values(), key=lambda hyp: hyp.tot_score)
 
     def remove(self, hyp: Hypothesis) -> None:
         """Remove a given hypothesis.
@@ -1629,7 +1656,7 @@ class HypothesisList(object):
         """
         ans = HypothesisList()
         for _, hyp in self._data.items():
-            if hyp.log_prob > threshold:
+            if hyp.tot_score > threshold:
                 ans.add(hyp)  # shallow copy
         return ans
 
@@ -1645,17 +1672,20 @@ class HypothesisList(object):
 
         if length_norm:
             hyps = sorted(
-                hyps, key=lambda h: h[1].log_prob / len(h[1].ys), reverse=True
+                hyps, key=lambda h: h[1].tot_score / len(h[1].ys), reverse=True
             )[:k]
         else:
-            hyps = sorted(hyps, key=lambda h: h[1].log_prob, reverse=True)[:k]
+            hyps = sorted(hyps, key=lambda h: h[1].tot_score, reverse=True)[:k]
 
         ans = HypothesisList(dict(hyps))
         return ans
 
-    def __contains__(self, key: str):
+    def __contains__(self, key: tuple):
         return key in self._data
 
+    def __getitem__(self, key: tuple):
+        return self._data[key]
+
     def __iter__(self):
         return iter(self._data.values())
 
@@ -1694,64 +1724,96 @@ def get_hyps_shape(hyps: List[HypothesisList]) -> k2.RaggedShape:
     return ans
 
 
-def _step_worker(log_probs, indexes, B, beam, blank_id):
+def _step_worker(
+    log_probs,
+    indexes,
+    B,
+    beam,
+    blank_id,
+    lm_scale: float = 0,
+    LODR_lm_scale: float = 0,
+    context_graph: Optional[ContextGraph] = None,
+):
     A = list(B)
     B = HypothesisList()
     for h in range(len(A)):
         hyp = A[h]
         for k in range(log_probs.size(0)):
             log_prob, index = log_probs[k], indexes[k]
-            if index == blank_id:
+            new_token = index.item()
+            update_prefix = False
+            new_hyp = hyp.clone()
+            if new_token == blank_id:
                 # Case 0: *a + ε => *a
                 #         *aε + ε => *a
                 # Prefix does not change, update log_prob of blank
-                new_hyp = Hypothesis(
+                new_hyp.log_prob_non_blank = torch.tensor(
-                    ys=hyp.ys[:],
-                    log_prob_non_blank=torch.tensor(
                     [float("-inf")], dtype=torch.float32
-                    ),
-                    log_prob_blank=hyp.log_prob + log_prob,
                 )
+                new_hyp.log_prob_blank = hyp.log_prob + log_prob
                 B.add(new_hyp)
-            elif len(hyp.ys) > 0 and hyp.ys[-1] == index:
+            elif len(hyp.ys) > 0 and hyp.ys[-1] == new_token:
                 # Case 1: *a + a => *a
                 # Prefix does not change, update log_prob of non_blank
-                new_hyp = Hypothesis(
+                new_hyp.log_prob_non_blank = hyp.log_prob_non_blank + log_prob
-                    ys=hyp.ys[:],
+                new_hyp.log_prob_blank = torch.tensor(
-                    log_prob_non_blank=hyp.log_prob_non_blank + log_prob,
+                    [float("-inf")], dtype=torch.float32
-                    log_prob_blank=torch.tensor([float("-inf")], dtype=torch.float32),
                 )
                 B.add(new_hyp)
 
                 # Case 2: *aε + a => *aa
                 # Prefix changes, update log_prob of blank
-                new_hyp = Hypothesis(
+                new_hyp = hyp.clone()
-                    ys=hyp.ys[:] + [index.item()],
+                # Caution: DO NOT use append, as clone is shallow copy
-                    log_prob_non_blank=hyp.log_prob_blank + log_prob,
+                new_hyp.ys = hyp.ys + [new_token]
-                    log_prob_blank=torch.tensor([float("-inf")], dtype=torch.float32),
+                new_hyp.log_prob_non_blank = hyp.log_prob_blank + log_prob
+                new_hyp.log_prob_blank = torch.tensor(
+                    [float("-inf")], dtype=torch.float32
                 )
-                B.add(new_hyp)
+                update_prefix = True
             else:
                 # Case 3: *a + b => *ab, *aε + b => *ab
                 # Prefix changes, update log_prob of non_blank
-                new_hyp = Hypothesis(
+                # Caution: DO NOT use append, as clone is shallow copy
-                    ys=hyp.ys[:] + [index.item()],
+                new_hyp.ys = hyp.ys + [new_token]
-                    log_prob_non_blank=hyp.log_prob + log_prob,
+                new_hyp.log_prob_non_blank = hyp.log_prob + log_prob
-                    log_prob_blank=torch.tensor([float("-inf")], dtype=torch.float32),
+                new_hyp.log_prob_blank = torch.tensor(
+                    [float("-inf")], dtype=torch.float32
                 )
+                update_prefix = True
+
+            if update_prefix:
+                lm_score = hyp.lm_score
+                if hyp.lm_log_probs is not None:
+                    lm_score += hyp.lm_log_probs[new_token] * lm_scale
+                    new_hyp.lm_log_probs = None
+
+                if context_graph is not None and hyp.context_state is not None:
+                    context_score, new_context_state = context_graph.forward_one_step(
+                        hyp.context_state, new_token
+                    )
+                    lm_score += context_score
+                    new_hyp.context_state = new_context_state
+
+                if hyp.LODR_state is not None:
+                    state_cost = hyp.LODR_state.forward_one_step(new_token)
+                    # calculate the score of the latest token
+                    current_ngram_score = state_cost.lm_score - hyp.LODR_state.lm_score
+                    assert current_ngram_score <= 0.0, (
+                        state_cost.lm_score,
+                        hyp.LODR_state.lm_score,
+                    )
+                    lm_score += LODR_lm_scale * current_ngram_score
+                    new_hyp.LODR_state = state_cost
+
+                new_hyp.lm_score = lm_score
                 B.add(new_hyp)
     B = B.topk(beam)
     return B
 
 
 def _batch_worker(topk_values, topk_indexes, B, encoder_out_lens, beam, blank_id):
-    B.add(
+    B.add(Hypothesis())
-        Hypothesis(
-            ys=[],
-            log_prob_non_blank=torch.tensor([float("-inf")], dtype=torch.float32),
-            log_prob_blank=torch.zeros(1, dtype=torch.float32),
-        )
-    )
     for j in range(encoder_out_lens):
         log_probs, indexes = topk_values[j], topk_indexes[j]
         B = _step_worker(log_probs, indexes, B, beam, blank_id)
@@ -1763,11 +1825,11 @@ def ctc_prefix_beam_search(
     encoder_out_lens: torch.Tensor,
     beam: int = 4,
     blank_id: int = 0,
-    context_graph: Optional[ContextGraph] = None,
     process_pool: Optional[Pool] = None,
     return_nbest: Optional[bool] = False,
 ) -> Union[List[List[int]], List[HypothesisList]]:
     batch_size, num_frames, vocab_size = ctc_output.shape
+
     # TODO: using a larger beam for first pass pruning
     topk_values, topk_indexes = ctc_output.topk(beam)  # (B, T, beam)
     topk_values = topk_values.cpu()
@@ -1800,6 +1862,136 @@ def ctc_prefix_beam_search(
         return [hyp.ys for hyp in best_hyps]
 
 
+def ctc_prefix_beam_search_shallow_fussion(
+    ctc_output: torch.Tensor,
+    encoder_out_lens: torch.Tensor,
+    beam: int = 4,
+    blank_id: int = 0,
+    LODR_lm: Optional[NgramLm] = None,
+    LODR_lm_scale: Optional[float] = 0,
+    LM: Optional[LmScorer] = None,
+    context_graph: Optional[ContextGraph] = None,
+) -> List[List[int]]:
+    batch_size, num_frames, vocab_size = ctc_output.shape
+    # TODO: using a larger beam for first pass pruning
+    topk_values, topk_indexes = ctc_output.topk(beam)  # (B, T, beam)
+    topk_values = topk_values.cpu()
+    topk_indexes = topk_indexes.cpu()
+    encoder_out_lens = encoder_out_lens.tolist()
+    device = ctc_output.device
+
+    lm_scale = 0
+    init_scores = None
+    init_states = None
+
+    if LM is not None:
+        lm_scale = LM.lm_scale
+        sos_id = getattr(LM, "sos_id", 1)
+        # get initial lm score and lm state by scoring the "sos" token
+        sos_token = torch.tensor([[sos_id]]).to(torch.int64).to(device)
+        lens = torch.tensor([1]).to(device)
+        init_scores, init_states = LM.score_token(sos_token, lens)
+        init_scores, init_states = init_scores.cpu(), (
+            init_states[0].cpu(),
+            init_states[1].cpu(),
+        )
+
+    B = [HypothesisList() for _ in range(batch_size)]
+    for i in range(batch_size):
+        B[i].add(
+            Hypothesis(
+                ys=[],
+                log_prob_non_blank=torch.tensor([float("-inf")], dtype=torch.float32),
+                log_prob_blank=torch.zeros(1, dtype=torch.float32),
+                lm_score=torch.zeros(1, dtype=torch.float32),
+                state=init_states,
+                lm_log_probs=None if init_scores is None else init_scores.reshape(-1),
+                LODR_state=None if LODR_lm is None else NgramLmStateCost(LODR_lm),
+                context_state=None if context_graph is None else context_graph.root,
+            )
+        )
+    for j in range(num_frames):
+        for i in range(batch_size):
+            if j < encoder_out_lens[i]:
+                log_probs, indexes = topk_values[i][j], topk_indexes[i][j]
+                B[i] = _step_worker(
+                    log_probs,
+                    indexes,
+                    B[i],
+                    beam,
+                    blank_id,
+                    lm_scale=lm_scale,
+                    LODR_lm_scale=LODR_lm_scale,
+                    context_graph=context_graph,
+                )
+        if LM is None:
+            continue
+        # update lm_score
+        token_list = []  # a list of list
+        hs = []
+        cs = []
+        indexes = []  # (batch_idx, key)
+        for batch_idx, hyps in enumerate(B):
+            for hyp in hyps:
+                if hyp.lm_log_probs is None:
+                    if LM.lm_type == "rnn":
+                        token_list.append([hyp.ys[-1]])
+                        # store the LSTM states
+                        hs.append(hyp.state[0])
+                        cs.append(hyp.state[1])
+                    else:
+                        # for transformer LM
+                        token_list.append([sos_id] + hyp.ys[:])
+                    indexes.append((batch_idx, hyp.key))
+        if len(token_list) != 0:
+            x_lens = torch.tensor([len(tokens) for tokens in token_list]).to(device)
+            if LM.lm_type == "rnn":
+                tokens_to_score = (
+                    torch.tensor(token_list).to(torch.int64).to(device).reshape(-1, 1)
+                )
+                hs = torch.cat(hs, dim=1).to(device)
+                cs = torch.cat(cs, dim=1).to(device)
+                state = (hs, cs)
+            else:
+                # for transformer LM
+                tokens_list = [torch.tensor(tokens) for tokens in token_list]
+                tokens_to_score = (
+                    torch.nn.utils.rnn.pad_sequence(
+                        tokens_list, batch_first=True, padding_value=0.0
+                    )
+                    .to(device)
+                    .to(torch.int64)
+                )
+                state = None
+
+            scores, lm_states = LM.score_token(tokens_to_score, x_lens, state)
+            scores, lm_states = scores.cpu(), (lm_states[0].cpu(), lm_states[1].cpu())
+            assert scores.size(0) == len(indexes), (scores.size(0), len(indexes))
+            for i in range(scores.size(0)):
+                batch_idx, key = indexes[i]
+                B[batch_idx][key].lm_log_probs = scores[i]
+                if LM.lm_type == "rnn":
+                    state = (
+                        lm_states[0][:, i, :].unsqueeze(1),
+                        lm_states[1][:, i, :].unsqueeze(1),
+                    )
+                    B[batch_idx][key].state = state
+
+    # finalize context_state, if the matched contexts do not reach final state
+    # we need to add the score on the corresponding backoff arc
+    if context_graph is not None:
+        for hyps in B:
+            for hyp in hyps:
+                context_score, new_context_state = context_graph.finalize(
+                    hyp.context_state
+                )
+                hyp.lm_score += context_score
+                hyp.context_state = new_context_state
+
+    best_hyps = [b.get_most_probable() for b in B]
+    return [hyp.ys for hyp in best_hyps]
+
+
 def ctc_prefix_beam_search_attention_decoder_rescoring(
     ctc_output: torch.Tensor,
     attention_decoder: torch.nn.Module,

icefall/utils.py

@@ -19,8 +19,10 @@
 
 import argparse
 import collections
+import json
 import logging
 import os
+import pathlib
 import re
 import subprocess
 from collections import defaultdict
@@ -178,6 +180,15 @@ class AttributeDict(dict):
             return
         raise AttributeError(f"No such attribute '{key}'")
 
+    def __str__(self, indent: int = 2):
+        tmp = {}
+        for k, v in self.items():
+            # PosixPath is ont JSON serializable
+            if isinstance(v, pathlib.Path) or isinstance(v, torch.device):
+                v = str(v)
+            tmp[k] = v
+        return json.dumps(tmp, indent=indent, sort_keys=True)
+
 
 def encode_supervisions(
     supervisions: dict,