Add LODR support to online and offline recognizers (#2026)

This PR integrates LODR (Level-Ordered Deterministic Rescoring) support from Icefall into both online and offline recognizers, enabling LODR for LM shallow fusion and LM rescore.

- Extended OnlineLMConfig and OfflineLMConfig to include lodr_fst, lodr_scale, and lodr_backoff_id.
- Implemented LodrFst and LodrStateCost classes and wired them into RNN LM scoring in both online and offline code paths.
- Updated Python bindings, CLI entry points, examples, and CI test scripts to accept and exercise the new LODR options.

python-api-examples/online-decode-files.py

@@ -21,6 +21,22 @@ rm sherpa-onnx-streaming-zipformer-en-2023-06-26.tar.bz2
   ./sherpa-onnx-streaming-zipformer-en-2023-06-26/test_wavs/1.wav \
   ./sherpa-onnx-streaming-zipformer-en-2023-06-26/test_wavs/8k.wav
 
+or with RNN LM rescoring and LODR:
+
+./python-api-examples/online-decode-files.py \
+  --tokens=./sherpa-onnx-streaming-zipformer-en-2023-06-26/tokens.txt \
+  --encoder=./sherpa-onnx-streaming-zipformer-en-2023-06-26/encoder-epoch-99-avg-1-chunk-16-left-64.onnx \
+  --decoder=./sherpa-onnx-streaming-zipformer-en-2023-06-26/decoder-epoch-99-avg-1-chunk-16-left-64.onnx \
+  --joiner=./sherpa-onnx-streaming-zipformer-en-2023-06-26/joiner-epoch-99-avg-1-chunk-16-left-64.onnx \
+  --decoding-method=modified_beam_search \
+  --lm=/path/to/lm.onnx \
+  --lm-scale=0.1 \
+  --lodr-fst=/path/to/lodr.fst \
+  --lodr-scale=-0.1 \
+  ./sherpa-onnx-streaming-zipformer-en-2023-06-26/test_wavs/0.wav \
+  ./sherpa-onnx-streaming-zipformer-en-2023-06-26/test_wavs/1.wav \
+  ./sherpa-onnx-streaming-zipformer-en-2023-06-26/test_wavs/8k.wav
+
 (2) Streaming paraformer
 
 curl -SL -O https://github.com/k2-fsa/sherpa-onnx/releases/download/asr-models/sherpa-onnx-streaming-paraformer-bilingual-zh-en.tar.bz2
@@ -186,6 +202,22 @@ def get_args():
         """,
     )
 
+    parser.add_argument(
+        "--lodr-fst",
+        metavar="file",
+        type=str,
+        default="",
+        help="Path to LODR FST model. Used only when --lm is given.",
+    )
+
+    parser.add_argument(
+        "--lodr-scale",
+        metavar="lodr_scale",
+        type=float,
+        default=-0.1,
+        help="LODR scale for rescoring.Used only when --lodr_fst is given.",
+    )
+
     parser.add_argument(
         "--provider",
         type=str,
@@ -320,6 +352,8 @@ def main():
             max_active_paths=args.max_active_paths,
             lm=args.lm,
             lm_scale=args.lm_scale,
+            lodr_fst=args.lodr_fst,
+            lodr_scale=args.lodr_scale,
             hotwords_file=args.hotwords_file,
             hotwords_score=args.hotwords_score,
             modeling_unit=args.modeling_unit,