Add other decoding methods (nbest, nbest oracle, nbest LG) for wenetspeech pruned rnnt2 (#482)

* add other decoding methods for wenetspeech

* changes for RESULTS.md

* add ngram-lm-scale=0.35 results

* set ngram-lm-scale=0.35 as default

* Update README.md

* add nbest-scale for flie name

README.md

@@ -257,8 +257,8 @@ We provide some models for this recipe: [Pruned stateless RNN-T_2: Conformer enc
 |                      |  Dev  | Test-Net | Test-Meeting |
 |----------------------|-------|----------|--------------|
 |    greedy search     | 7.80  |  8.75    |  13.49       |
+| modified beam search| 7.76  |  8.71    |  13.41       |
 |   fast beam search   | 7.94  |  8.74    |  13.80       |
-| modified beam search | 7.76  |  8.71    |  13.41       |
 
 #### Pruned stateless RNN-T_5: Conformer encoder + Embedding decoder + k2 pruned RNN-T loss (trained with L subset)
 **Streaming**:

egs/wenetspeech/ASR/RESULTS.md

@@ -84,7 +84,10 @@ When training with the L subset, the CERs are
 |------------------------------------|-------|----------|--------------|------------------------------------------|
 |          greedy search             | 7.80  | 8.75     | 13.49        | --epoch 10, --avg 2, --max-duration 100  |
 | modified beam search (beam size 4) | 7.76  | 8.71     | 13.41        | --epoch 10, --avg 2, --max-duration 100  |
-| fast beam search (set as default)  | 7.94  | 8.74     | 13.80        | --epoch 10, --avg 2, --max-duration 1500 |
+| fast beam search (1best)  | 7.94  | 8.74     | 13.80        | --epoch 10, --avg 2, --max-duration 1500 |
+| fast beam search (nbest)  | 9.82  | 10.98    |     16.37   | --epoch 10, --avg 2, --max-duration 600 |
+| fast beam search (nbest oracle)  | 6.88 | 7.18    |     11.77   | --epoch 10, --avg 2, --max-duration 600 |
+| fast beam search (nbest LG, ngram_lm_scale=0.35)  | 8.83 | 9.88    |   15.47  | --epoch 10, --avg 2, --max-duration 600 |
 
 The training command for reproducing is given below:
 
@@ -131,7 +134,7 @@ avg=2
         --decoding-method modified_beam_search \
         --beam-size 4
 
-## fast beam search
+## fast beam search (1best)
 ./pruned_transducer_stateless2/decode.py \
         --epoch $epoch \
         --avg $avg \
@@ -142,6 +145,47 @@ avg=2
         --beam 4 \
         --max-contexts 4 \
         --max-states 8
+
+## fast beam search (nbest)
+./pruned_transducer_stateless2/decode.py \
+        --epoch 10 \
+        --avg 2 \
+        --exp-dir ./pruned_transducer_stateless2/exp \
+        --lang-dir data/lang_char \
+        --max-duration 600 \
+        --decoding-method fast_beam_search_nbest \
+        --beam 20.0 \
+        --max-contexts 8 \
+        --max-states 64 \
+        --num-paths 200 \
+        --nbest-scale 0.5
+
+## fast beam search (nbest oracle WER)
+./pruned_transducer_stateless2/decode.py \
+        --epoch 10 \
+        --avg 2 \
+        --exp-dir ./pruned_transducer_stateless2/exp \
+        --lang-dir data/lang_char \
+        --max-duration 600 \
+        --decoding-method fast_beam_search_nbest_oracle \
+        --beam 20.0 \
+        --max-contexts 8 \
+        --max-states 64 \
+        --num-paths 200 \
+        --nbest-scale 0.5
+
+## fast beam search (with LG)
+./pruned_transducer_stateless2/decode.py \
+        --epoch 10 \
+        --avg 2 \
+        --exp-dir ./pruned_transducer_stateless2/exp \
+        --lang-dir data/lang_char \
+        --max-duration 600 \
+        --decoding-method fast_beam_search_nbest_LG \
+        --ngram-lm-scale 0.35 \
+        --beam 20.0 \
+        --max-contexts 8 \
+        --max-states 64
 ```
 
 When training with the M subset, the CERs are

egs/wenetspeech/ASR/local/compile_lg.py

@@ -0,0 +1 @@
+../../../librispeech/ASR/local/compile_lg.py

egs/wenetspeech/ASR/prepare.sh

@@ -225,3 +225,34 @@ if [ $stage -le 16 ] && [ $stop_stage -ge 16 ]; then
       --lang-dir data/lang_char
   fi
 fi
+
+# If you don't want to use LG for decoding, the following steps are not necessary.
+if [ $stage -le 17 ] && [ $stop_stage -ge 17 ]; then
+  log "Stage 17: Prepare G"
+  # It will take about 20 minutes.
+  # We assume you have install kaldilm, if not, please install
+  # it using: pip install kaldilm
+  lang_char_dir=data/lang_char
+  if [ ! -f $lang_char_dir/3-gram.unpruned.arpa ]; then
+    python ./shared/make_kn_lm.py \
+      -ngram-order 3 \
+      -text $lang_char_dir/text_words_segmentation \
+      -lm $lang_char_dir/3-gram.unpruned.arpa
+  fi
+
+  mkdir -p data/lm
+  if [ ! -f data/lm/G_3_gram.fst.txt ]; then
+    # It is used in building LG
+    python3 -m kaldilm \
+      --read-symbol-table="$lang_char_dir/words.txt" \
+      --disambig-symbol='#0' \
+      --max-order=3 \
+      $lang_char_dir/3-gram.unpruned.arpa > data/lm/G_3_gram.fst.txt
+  fi
+fi
+
+if [ $stage -le 18 ] && [ $stop_stage -ge 18 ]; then
+  log "Stage 18: Compile LG"
+  lang_char_dir=data/lang_char
+  python ./local/compile_lg.py --lang-dir $lang_char_dir
+fi

egs/wenetspeech/ASR/pruned_transducer_stateless2/decode.py

@@ -37,7 +37,7 @@ When training with the L subset, usage:
         --decoding-method modified_beam_search \
         --beam-size 4
 
-(3) fast beam search
+(3) fast beam search (1best)
 ./pruned_transducer_stateless2/decode.py \
         --epoch 10 \
         --avg 2 \
@@ -48,6 +48,46 @@ When training with the L subset, usage:
         --beam 4 \
         --max-contexts 4 \
         --max-states 8
+
+(4) fast beam search (nbest)
+./pruned_transducer_stateless2/decode.py \
+    --epoch 10 \
+    --avg 2 \
+    --exp-dir ./pruned_transducer_stateless2/exp \
+    --lang-dir data/lang_char \
+    --max-duration 600 \
+    --decoding-method fast_beam_search_nbest \
+    --beam 20.0 \
+    --max-contexts 8 \
+    --max-states 64 \
+    --num-paths 200 \
+    --nbest-scale 0.5
+
+(5) fast beam search (nbest oracle WER)
+./pruned_transducer_stateless2/decode.py \
+    --epoch 10 \
+    --avg 2 \
+    --exp-dir ./pruned_transducer_stateless2/exp \
+    --lang-dir data/lang_char \
+    --max-duration 600 \
+    --decoding-method fast_beam_search_nbest_oracle \
+    --beam 20.0 \
+    --max-contexts 8 \
+    --max-states 64 \
+    --num-paths 200 \
+    --nbest-scale 0.5
+
+(6) fast beam search (with LG)
+./pruned_transducer_stateless2/decode.py \
+    --epoch 10 \
+    --avg 2 \
+    --exp-dir ./pruned_transducer_stateless2/exp \
+    --lang-dir data/lang_char \
+    --max-duration 600 \
+    --decoding-method fast_beam_search_nbest_LG \
+    --beam 20.0 \
+    --max-contexts 8 \
+    --max-states 64
 """
 
 
@@ -63,6 +103,9 @@ import torch.nn as nn
 from asr_datamodule import WenetSpeechAsrDataModule
 from beam_search import (
     beam_search,
+    fast_beam_search_nbest,
+    fast_beam_search_nbest_LG,
+    fast_beam_search_nbest_oracle,
     fast_beam_search_one_best,
     greedy_search,
     greedy_search_batch,
@@ -70,6 +113,7 @@ from beam_search import (
 )
 from train import get_params, get_transducer_model
 
+from icefall.char_graph_compiler import CharCtcTrainingGraphCompiler
 from icefall.checkpoint import (
     average_checkpoints,
     find_checkpoints,
@@ -151,6 +195,11 @@ def get_parser():
           - beam_search
           - modified_beam_search
           - fast_beam_search
+          - fast_beam_search_nbest
+          - fast_beam_search_nbest_oracle
+          - fast_beam_search_nbest_LG
+        If you use fast_beam_search_nbest_LG, you have to
+        specify `--lang-dir`, which should contain `LG.pt`.
         """,
     )
 
@@ -173,6 +222,16 @@ def get_parser():
         Used only when --decoding-method is fast_beam_search""",
     )
 
+    parser.add_argument(
+        "--ngram-lm-scale",
+        type=float,
+        default=0.35,
+        help="""
+        Used only when --decoding_method is fast_beam_search_nbest_LG.
+        It specifies the scale for n-gram LM scores.
+        """,
+    )
+
     parser.add_argument(
         "--max-contexts",
         type=int,
@@ -204,6 +263,24 @@ def get_parser():
         Used only when --decoding_method is greedy_search""",
     )
 
+    parser.add_argument(
+        "--num-paths",
+        type=int,
+        default=200,
+        help="""Number of paths for nbest decoding.
+        Used only when the decoding method is fast_beam_search_nbest,
+        fast_beam_search_nbest_LG, and fast_beam_search_nbest_oracle""",
+    )
+
+    parser.add_argument(
+        "--nbest-scale",
+        type=float,
+        default=0.5,
+        help="""Scale applied to lattice scores when computing nbest paths.
+        Used only when the decoding method is fast_beam_search_nbest,
+        fast_beam_search_nbest_LG, and fast_beam_search_nbest_oracle""",
+    )
+
     return parser
 
 
@@ -211,6 +288,7 @@ def decode_one_batch(
     params: AttributeDict,
     model: nn.Module,
     lexicon: Lexicon,
+    graph_compiler: CharCtcTrainingGraphCompiler,
     batch: dict,
     decoding_graph: Optional[k2.Fsa] = None,
 ) -> Dict[str, List[List[str]]]:
@@ -267,6 +345,50 @@ def decode_one_batch(
         )
         for i in range(encoder_out.size(0)):
             hyps.append([lexicon.token_table[idx] for idx in hyp_tokens[i]])
+    elif params.decoding_method == "fast_beam_search_nbest_LG":
+        hyp_tokens = fast_beam_search_nbest_LG(
+            model=model,
+            decoding_graph=decoding_graph,
+            encoder_out=encoder_out,
+            encoder_out_lens=encoder_out_lens,
+            beam=params.beam,
+            max_contexts=params.max_contexts,
+            max_states=params.max_states,
+            num_paths=params.num_paths,
+            nbest_scale=params.nbest_scale,
+        )
+        for hyp in hyp_tokens:
+            sentence = "".join([lexicon.word_table[i] for i in hyp])
+            hyps.append(list(sentence))
+    elif params.decoding_method == "fast_beam_search_nbest":
+        hyp_tokens = fast_beam_search_nbest(
+            model=model,
+            decoding_graph=decoding_graph,
+            encoder_out=encoder_out,
+            encoder_out_lens=encoder_out_lens,
+            beam=params.beam,
+            max_contexts=params.max_contexts,
+            max_states=params.max_states,
+            num_paths=params.num_paths,
+            nbest_scale=params.nbest_scale,
+        )
+        for i in range(encoder_out.size(0)):
+            hyps.append([lexicon.token_table[idx] for idx in hyp_tokens[i]])
+    elif params.decoding_method == "fast_beam_search_nbest_oracle":
+        hyp_tokens = fast_beam_search_nbest_oracle(
+            model=model,
+            decoding_graph=decoding_graph,
+            encoder_out=encoder_out,
+            encoder_out_lens=encoder_out_lens,
+            beam=params.beam,
+            max_contexts=params.max_contexts,
+            max_states=params.max_states,
+            num_paths=params.num_paths,
+            ref_texts=graph_compiler.texts_to_ids(supervisions["text"]),
+            nbest_scale=params.nbest_scale,
+        )
+        for i in range(encoder_out.size(0)):
+            hyps.append([lexicon.token_table[idx] for idx in hyp_tokens[i]])
     elif (
         params.decoding_method == "greedy_search"
         and params.max_sym_per_frame == 1
@@ -331,6 +453,7 @@ def decode_dataset(
     params: AttributeDict,
     model: nn.Module,
     lexicon: Lexicon,
+    graph_compiler: CharCtcTrainingGraphCompiler,
     decoding_graph: Optional[k2.Fsa] = None,
 ) -> Dict[str, List[Tuple[List[str], List[str]]]]:
     """Decode dataset.
@@ -373,6 +496,7 @@ def decode_dataset(
             params=params,
             model=model,
             lexicon=lexicon,
+            graph_compiler=graph_compiler,
             decoding_graph=decoding_graph,
             batch=batch,
         )
@@ -454,6 +578,9 @@ def main():
         "greedy_search",
         "beam_search",
         "fast_beam_search",
+        "fast_beam_search_nbest",
+        "fast_beam_search_nbest_LG",
+        "fast_beam_search_nbest_oracle",
         "modified_beam_search",
     )
     params.res_dir = params.exp_dir / params.decoding_method
@@ -463,6 +590,13 @@ def main():
         params.suffix += f"-beam-{params.beam}"
         params.suffix += f"-max-contexts-{params.max_contexts}"
         params.suffix += f"-max-states-{params.max_states}"
+        if params.decoding_method == "fast_beam_search_nbest_LG":
+            params.suffix += f"-ngram-lm-scale-{params.ngram_lm_scale}"
+        if (
+            params.decoding_method == "fast_beam_search_nbest"
+            or params.decoding_method == "fast_beam_search_nbest_oracle"
+        ):
+            params.suffix += f"-nbest-scale-{params.nbest_scale}"
     elif "beam_search" in params.decoding_method:
         params.suffix += f"-beam-{params.beam_size}"
     else:
@@ -482,6 +616,11 @@ def main():
     params.blank_id = lexicon.token_table["<blk>"]
     params.vocab_size = max(lexicon.tokens) + 1
 
+    graph_compiler = CharCtcTrainingGraphCompiler(
+        lexicon=lexicon,
+        device=device,
+    )
+
     logging.info(params)
 
     logging.info("About to create model")
@@ -513,8 +652,18 @@ def main():
     model.eval()
     model.device = device
 
-    if params.decoding_method == "fast_beam_search":
+    if "fast_beam_search" in params.decoding_method:
-        decoding_graph = k2.trivial_graph(params.vocab_size - 1, device=device)
+        if params.decoding_method == "fast_beam_search_nbest_LG":
+            lg_filename = params.lang_dir + "/LG.pt"
+            logging.info(f"Loading {lg_filename}")
+            decoding_graph = k2.Fsa.from_dict(
+                torch.load(lg_filename, map_location=device)
+            )
+            decoding_graph.scores *= params.ngram_lm_scale
+        else:
+            decoding_graph = k2.trivial_graph(
+                params.vocab_size - 1, device=device
+            )
     else:
         decoding_graph = None
 
@@ -610,6 +759,7 @@ def main():
             params=params,
             model=model,
             lexicon=lexicon,
+            graph_compiler=graph_compiler,
             decoding_graph=decoding_graph,
         )
         save_results(