online-transducer: reset the encoder toghter with 2 previous output symbols (non-blank) (#2129)

* online-transducer: reset the encoder toghter with 2 previous output symbols (non-blank)

- added `reset_encoder` boolean member into the OnlineRecognizerConfig class
- by default the encoder is not reset

* pybind11, adding empty symbols for disabled modules (tts, diarization)

* reset_encoder, add default value (false) [pybind11]

sherpa-onnx/csrc/online-recognizer-transducer-impl.h

@@ -382,14 +382,13 @@ class OnlineRecognizerTransducerImpl : public OnlineRecognizerImpl {
       }
     }
 
-    // reset encoder states
-    // s->SetStates(model_->GetEncoderInitStates());
-
     auto r = decoder_->GetEmptyResult();
     auto last_result = s->GetResult();
-    // if last result is not empty, then
-    // truncate all last hyps and save as the context for next result
+
     if (static_cast<int32_t>(last_result.tokens.size()) > context_size) {
+      // if last result is not empty, then
+      // truncate all last hyps and save as the 'ys' context for next result
+      // (the encoder state buffers are kept)
       for (const auto &it : last_result.hyps) {
         auto h = it.second;
         r.hyps.Add({std::vector<int64_t>(h.ys.end() - context_size,
@@ -399,6 +398,11 @@ class OnlineRecognizerTransducerImpl : public OnlineRecognizerImpl {
 
       r.tokens = std::vector<int64_t> (last_result.tokens.end() - context_size,
                                        last_result.tokens.end());
+    } else {
+      if(config_.reset_encoder) {
+        // reset encoder states, use blanks as 'ys' context
+        s->SetStates(model_->GetEncoderInitStates());
+      }
     }
 
     // but reset all contextual biasing graph states to root

sherpa-onnx/csrc/online-recognizer.cc

@@ -121,6 +121,10 @@ void OnlineRecognizerConfig::Register(ParseOptions *po) {
       "rule-fars", &rule_fars,
       "If not empty, it specifies fst archives for inverse text normalization. "
       "If there are multiple archives, they are separated by a comma.");
+
+  po->Register("reset-encoder", &reset_encoder,
+               "True to reset encoder_state on an endpoint after empty segment."
+               "Done in `Reset()` method, after an endpoint was detected.");
 }
 
 bool OnlineRecognizerConfig::Validate() const {
@@ -198,7 +202,8 @@ std::string OnlineRecognizerConfig::ToString() const {
   os << "blank_penalty=" << blank_penalty << ", ";
   os << "temperature_scale=" << temperature_scale << ", ";
   os << "rule_fsts=\"" << rule_fsts << "\", ";
-  os << "rule_fars=\"" << rule_fars << "\")";
+  os << "rule_fars=\"" << rule_fars << "\", ";
+  os << "reset_encoder=\"" << (reset_encoder ? "True" : "False") << "\")";
 
   return os.str();
 }

sherpa-onnx/csrc/online-recognizer.h

@@ -79,6 +79,7 @@ struct OnlineRecognizerConfig {
   OnlineLMConfig lm_config;
   EndpointConfig endpoint_config;
   OnlineCtcFstDecoderConfig ctc_fst_decoder_config;
+
   bool enable_endpoint = true;
 
   std::string decoding_method = "greedy_search";
@@ -101,6 +102,11 @@ struct OnlineRecognizerConfig {
   // If there are multiple FST archives, they are applied from left to right.
   std::string rule_fars;
 
+  // True to reset encoder_state on an endpoint after empty segment.
+  // Done in `Reset()` method, after an endpoint was detected,
+  // currently only in `OnlineRecognizerTransducerImpl`.
+  bool reset_encoder = false;
+
   /// used only for modified_beam_search, if hotwords_buf is non-empty,
   /// the hotwords will be loaded from the buffered string instead of from the
   /// "hotwords_file"
@@ -116,7 +122,8 @@ struct OnlineRecognizerConfig {
       bool enable_endpoint, const std::string &decoding_method,
       int32_t max_active_paths, const std::string &hotwords_file,
       float hotwords_score, float blank_penalty, float temperature_scale,
-      const std::string &rule_fsts, const std::string &rule_fars)
+      const std::string &rule_fsts, const std::string &rule_fars,
+      bool reset_encoder)
       : feat_config(feat_config),
         model_config(model_config),
         lm_config(lm_config),
@@ -130,7 +137,8 @@ struct OnlineRecognizerConfig {
         blank_penalty(blank_penalty),
         temperature_scale(temperature_scale),
         rule_fsts(rule_fsts),
-        rule_fars(rule_fars) {}
+        rule_fars(rule_fars),
+        reset_encoder(reset_encoder) {}
 
   void Register(ParseOptions *po);
   bool Validate() const;