Add RNN LM rescore for offline ASR with modified_beam_search (#125)

sherpa-onnx/csrc/offline-transducer-modified-beam-search-decoder.h

@@ -0,0 +1,41 @@
+// sherpa-onnx/csrc/offline-transducer-modified-beam-search-decoder.h
+//
+// Copyright (c)  2023  Xiaomi Corporation
+
+#ifndef SHERPA_ONNX_CSRC_OFFLINE_TRANSDUCER_MODIFIED_BEAM_SEARCH_DECODER_H_
+#define SHERPA_ONNX_CSRC_OFFLINE_TRANSDUCER_MODIFIED_BEAM_SEARCH_DECODER_H_
+
+#include <vector>
+
+#include "sherpa-onnx/csrc/offline-lm.h"
+#include "sherpa-onnx/csrc/offline-transducer-decoder.h"
+#include "sherpa-onnx/csrc/offline-transducer-model.h"
+
+namespace sherpa_onnx {
+
+class OfflineTransducerModifiedBeamSearchDecoder
+    : public OfflineTransducerDecoder {
+ public:
+  OfflineTransducerModifiedBeamSearchDecoder(OfflineTransducerModel *model,
+                                             OfflineLM *lm,
+                                             int32_t max_active_paths,
+                                             float lm_scale)
+      : model_(model),
+        lm_(lm),
+        max_active_paths_(max_active_paths),
+        lm_scale_(lm_scale) {}
+
+  std::vector<OfflineTransducerDecoderResult> Decode(
+      Ort::Value encoder_out, Ort::Value encoder_out_length) override;
+
+ private:
+  OfflineTransducerModel *model_;  // Not owned
+  OfflineLM *lm_;                  // Not owned; may be nullptr
+
+  int32_t max_active_paths_;
+  float lm_scale_;  // used only when lm_ is not nullptr
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_OFFLINE_TRANSDUCER_MODIFIED_BEAM_SEARCH_DECODER_H_