Implement context biasing with a Aho Corasick automata (#145)

* Implement context graph

* Modify the interface to support context biasing

* Support context biasing in modified beam search; add python wrapper

* Support context biasing in python api example

* Minor fixes

* Fix context graph

* Minor fixes

* Fix tests

* Fix style

* Fix style

* Fix comments

* Minor fixes

* Add missing header

* Replace std::shared_ptr with std::unique_ptr for effciency

* Build graph in constructor

* Fix comments

* Minor fixes

* Fix docs

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

@@ -26,7 +26,8 @@ class OfflineTransducerModifiedBeamSearchDecoder
         lm_scale_(lm_scale) {}
 
   std::vector<OfflineTransducerDecoderResult> Decode(
-      Ort::Value encoder_out, Ort::Value encoder_out_length) override;
+      Ort::Value encoder_out, Ort::Value encoder_out_length,
+      OfflineStream **ss = nullptr, int32_t n = 0) override;
 
  private:
   OfflineTransducerModel *model_;  // Not owned