Add online transducer decoder (#27)

sherpa-onnx/csrc/online-transducer-decoder.h

@@ -0,0 +1,52 @@
+// sherpa/csrc/online-transducer-decoder.h
+//
+// Copyright (c)  2023  Xiaomi Corporation
+
+#ifndef SHERPA_ONNX_CSRC_ONLINE_TRANSDUCER_DECODER_H_
+#define SHERPA_ONNX_CSRC_ONLINE_TRANSDUCER_DECODER_H_
+
+#include <vector>
+
+#include "onnxruntime_cxx_api.h"  // NOLINT
+
+namespace sherpa_onnx {
+
+struct OnlineTransducerDecoderResult {
+  /// The decoded token IDs so far
+  std::vector<int64_t> tokens;
+};
+
+class OnlineTransducerDecoder {
+ public:
+  virtual ~OnlineTransducerDecoder() = default;
+
+  /* Return an empty result.
+   *
+   * To simplify the decoding code, we add `context_size` blanks
+   * to the beginning of the decoding result, which will be
+   * stripped by calling `StripPrecedingBlanks()`.
+   */
+  virtual OnlineTransducerDecoderResult GetEmptyResult() = 0;
+
+  /** Strip blanks added by `GetEmptyResult()`.
+   *
+   * @param r It is changed in-place.
+   */
+  virtual void StripLeadingBlanks(OnlineTransducerDecoderResult * /*r*/) {}
+
+  /** Run transducer beam search given the output from the encoder model.
+   *
+   * @param encoder_out A 3-D tensor of shape (N, T, joiner_dim)
+   * @param result  It is modified in-place.
+   *
+   * @note There is no need to pass encoder_out_length here since for the
+   * online decoding case, each utterance has the same number of frames
+   * and there are no paddings.
+   */
+  virtual void Decode(Ort::Value encoder_out,
+                      std::vector<OnlineTransducerDecoderResult> *result) = 0;
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_ONLINE_TRANSDUCER_DECODER_H_