Support contextual-biasing for streaming model (#184)

* Support contextual-biasing for streaming model

* The whole pipeline runs normally

* Fix comments

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

@@ -9,6 +9,7 @@
 
 #include "onnxruntime_cxx_api.h"  // NOLINT
 #include "sherpa-onnx/csrc/hypothesis.h"
+#include "sherpa-onnx/csrc/macros.h"
 
 namespace sherpa_onnx {
 
@@ -45,6 +46,7 @@ struct OnlineTransducerDecoderResult {
       OnlineTransducerDecoderResult &&other);
 };
 
+class OnlineStream;
 class OnlineTransducerDecoder {
  public:
   virtual ~OnlineTransducerDecoder() = default;
@@ -76,6 +78,26 @@ class OnlineTransducerDecoder {
   virtual void Decode(Ort::Value encoder_out,
                       std::vector<OnlineTransducerDecoderResult> *result) = 0;
 
+  /** Run transducer beam search given the output from the encoder model.
+   *
+   * Note: Currently this interface is for contextual-biasing feature which
+   *       needs a ContextGraph owned by the OnlineStream.
+   *
+   * @param encoder_out A 3-D tensor of shape (N, T, joiner_dim)
+   * @param ss  A list of OnlineStreams.
+   * @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, OnlineStream **ss,
+                      std::vector<OnlineTransducerDecoderResult> *result) {
+    SHERPA_ONNX_LOGE(
+        "This interface is for OnlineTransducerModifiedBeamSearchDecoder.");
+    exit(-1);
+  }
+
   // used for endpointing. We need to keep decoder_out after reset
   virtual void UpdateDecoderOut(OnlineTransducerDecoderResult *result) {}
 };