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