offline transducer: treat unk as blank (#1005)

Co-authored-by: chungyi.li <chungyi.li@ailabs.tw>

sherpa-onnx/csrc/offline-transducer-modified-beam-search-decoder.cc

@@ -131,8 +131,9 @@ OfflineTransducerModifiedBeamSearchDecoder::Decode(
 
         float context_score = 0;
         auto context_state = new_hyp.context_state;
-        if (new_token != 0) {
-          // blank id is fixed to 0
+        // blank is hardcoded to 0
+        // also, it treats unk as blank
+        if (new_token != 0 && new_token != unk_id_) {
           new_hyp.ys.push_back(new_token);
           new_hyp.timestamps.push_back(t);
           if (context_graphs[i] != nullptr) {