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

sherpa-onnx/csrc/online-lm.h

@@ -0,0 +1,64 @@
+// sherpa-onnx/csrc/online-lm.h
+//
+// Copyright (c)  2023  Xiaomi Corporation
+
+#ifndef SHERPA_ONNX_CSRC_ONLINE_LM_H_
+#define SHERPA_ONNX_CSRC_ONLINE_LM_H_
+
+#include <memory>
+#include <utility>
+#include <vector>
+
+#include "onnxruntime_cxx_api.h"  // NOLINT
+#include "sherpa-onnx/csrc/hypothesis.h"
+#include "sherpa-onnx/csrc/online-lm-config.h"
+
+namespace sherpa_onnx {
+
+class OnlineLM {
+ public:
+  virtual ~OnlineLM() = default;
+
+  static std::unique_ptr<OnlineLM> Create(const OnlineLMConfig &config);
+
+  virtual std::vector<Ort::Value> GetInitStates() = 0;
+
+  /** Rescore a batch of sentences.
+   *
+   * @param x A 2-D tensor of shape (N, L) with data type int64.
+   * @param y A 2-D tensor of shape (N, L) with data type int64.
+   * @param states It contains the states for the LM model
+   * @return Return a pair containingo
+   *          - negative loglike
+   *          - updated states
+   *
+   * Caution: It returns negative log likelihood (nll), not log likelihood
+   */
+  std::pair<Ort::Value, std::vector<Ort::Value>> Ort::Value Rescore(
+      Ort::Value x, Ort::Value y, std::vector<Ort::Value> states) = 0;
+
+  // This function updates hyp.lm_lob_prob of hyps.
+  //
+  // @param scale LM score
+  // @param context_size Context size of the transducer decoder model
+  // @param hyps It is changed in-place.
+  void ComputeLMScore(float scale, int32_t context_size,
+                      std::vector<Hypotheses> *hyps);
+  /** TODO(fangjun):
+   *
+   * 1. Add two fields to Hypothesis
+   *      (a) int32_t lm_cur_pos = 0; number of scored tokens so far
+   *      (b) std::vector<Ort::Value> lm_states;
+   * 2. When we want to score a hypothesis, we construct x and y as follows:
+   *
+   *      std::vector x = {hyp.ys.begin() + context_size + lm_cur_pos,
+   *                       hyp.ys.end() - 1};
+   *      std::vector y = {hyp.ys.begin() + context_size + lm_cur_pos + 1
+   *                       hyp.ys.end()};
+   *       hyp.lm_cur_pos += hyp.ys.size() - context_size - lm_cur_pos;
+   */
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_ONLINE_LM_H_