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.h

@@ -19,12 +19,13 @@ class OfflineTransducerModifiedBeamSearchDecoder
   OfflineTransducerModifiedBeamSearchDecoder(OfflineTransducerModel *model,
                                              OfflineLM *lm,
                                              int32_t max_active_paths,
-                                             float lm_scale,
+                                             float lm_scale, int32_t unk_id,
                                              float blank_penalty)
       : model_(model),
         lm_(lm),
         max_active_paths_(max_active_paths),
         lm_scale_(lm_scale),
+        unk_id_(unk_id),
         blank_penalty_(blank_penalty) {}
 
   std::vector<OfflineTransducerDecoderResult> Decode(
@@ -37,6 +38,7 @@ class OfflineTransducerModifiedBeamSearchDecoder
 
   int32_t max_active_paths_;
   float lm_scale_;  // used only when lm_ is not nullptr
+  int32_t unk_id_;
   float blank_penalty_;
 };