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/CMakeLists.txt

@@ -12,6 +12,7 @@ endif()
 
 set(sources
   cat.cc
+  context-graph.cc
   endpoint.cc
   features.cc
   file-utils.cc
@@ -248,6 +249,7 @@ endif()
 if(SHERPA_ONNX_ENABLE_TESTS)
   set(sherpa_onnx_test_srcs
     cat-test.cc
+    context-graph-test.cc
     packed-sequence-test.cc
     pad-sequence-test.cc
     slice-test.cc