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-stream.h

@@ -10,6 +10,7 @@
 #include <string>
 #include <vector>
 
+#include "sherpa-onnx/csrc/context-graph.h"
 #include "sherpa-onnx/csrc/parse-options.h"
 
 namespace sherpa_onnx {
@@ -66,7 +67,8 @@ struct OfflineFeatureExtractorConfig {
 
 class OfflineStream {
  public:
-  explicit OfflineStream(const OfflineFeatureExtractorConfig &config = {});
+  explicit OfflineStream(const OfflineFeatureExtractorConfig &config = {},
+                         ContextGraphPtr context_graph = nullptr);
   ~OfflineStream();
 
   /**
@@ -96,6 +98,9 @@ class OfflineStream {
   /** Get the recognition result of this stream */
   const OfflineRecognitionResult &GetResult() const;
 
+  /** Get the ContextGraph of this stream */
+  const ContextGraphPtr &GetContextGraph() const;
+
  private:
   class Impl;
   std::unique_ptr<Impl> impl_;