Track token scores (#571)

* add export of per-token scores (ys, lm, context)

- for best path of the modified-beam-search decoding of transducer

* refactoring JSON export of OnlineRecognitionResult, extending pybind11 API of OnlineRecognitionResult

* export per-token scores also for greedy-search (online-transducer)

- export un-scaled lm_probs (modified-beam search, online-transducer)
- polishing

* fill lm_probs/context_scores only if LM/ContextGraph is present (make Result smaller)

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

@@ -37,6 +37,10 @@ OnlineTransducerDecoderResult &OnlineTransducerDecoderResult::operator=(
   frame_offset = other.frame_offset;
   timestamps = other.timestamps;
 
+  ys_probs = other.ys_probs;
+  lm_probs = other.lm_probs;
+  context_scores = other.context_scores;
+
   return *this;
 }
 
@@ -60,6 +64,10 @@ OnlineTransducerDecoderResult &OnlineTransducerDecoderResult::operator=(
   frame_offset = other.frame_offset;
   timestamps = std::move(other.timestamps);
 
+  ys_probs = std::move(other.ys_probs);
+  lm_probs = std::move(other.lm_probs);
+  context_scores = std::move(other.context_scores);
+
   return *this;
 }