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-modified-beam-search-decoder.cc

@@ -59,6 +59,12 @@ void OnlineTransducerModifiedBeamSearchDecoder::StripLeadingBlanks(
   std::vector<int64_t> tokens(hyp.ys.begin() + context_size, hyp.ys.end());
   r->tokens = std::move(tokens);
   r->timestamps = std::move(hyp.timestamps);
+
+  // export per-token scores
+  r->ys_probs = std::move(hyp.ys_probs);
+  r->lm_probs = std::move(hyp.lm_probs);
+  r->context_scores = std::move(hyp.context_scores);
+
   r->num_trailing_blanks = hyp.num_trailing_blanks;
 }
 
@@ -180,6 +186,28 @@ void OnlineTransducerModifiedBeamSearchDecoder::Decode(
         new_hyp.log_prob = p_logprob[k] + context_score -
                            prev_lm_log_prob;  // log_prob only includes the
                                               // score of the transducer
+        // export the per-token log scores
+        if (new_token != 0 && new_token != unk_id_) {
+          const Hypothesis& prev_i = prev[hyp_index];
+          // subtract 'prev[i]' path scores, which were added before
+          // getting topk tokens
+          float y_prob = p_logprob[k] - prev_i.log_prob - prev_i.lm_log_prob;
+          new_hyp.ys_probs.push_back(y_prob);
+
+          if (lm_) {  // export only when LM is used
+            float lm_prob = new_hyp.lm_log_prob - prev_lm_log_prob;
+            if (lm_scale_ != 0.0) {
+              lm_prob /= lm_scale_;  // remove lm-scale
+            }
+            new_hyp.lm_probs.push_back(lm_prob);
+          }
+
+          // export only when `ContextGraph` is used
+          if (ss != nullptr && ss[b]->GetContextGraph() != nullptr) {
+            new_hyp.context_scores.push_back(context_score);
+          }
+        }
+
         hyps.Add(std::move(new_hyp));
       }  // for (auto k : topk)
       cur.push_back(std::move(hyps));