Refactor online recognizer (#250)

* Refactor online recognizer.

Make it easier to support other streaming models.

Note that it is a breaking change for the Python API.
`sherpa_onnx.OnlineRecognizer()` used before should be
replaced by `sherpa_onnx.OnlineRecognizer.from_transducer()`.

sherpa-onnx/csrc/online-recognizer-impl.h

@@ -0,0 +1,52 @@
+// sherpa-onnx/csrc/online-recognizer-impl.h
+//
+// Copyright (c)  2023  Xiaomi Corporation
+
+#ifndef SHERPA_ONNX_CSRC_ONLINE_RECOGNIZER_IMPL_H_
+#define SHERPA_ONNX_CSRC_ONLINE_RECOGNIZER_IMPL_H_
+
+#include <memory>
+#include <vector>
+
+#include "sherpa-onnx/csrc/macros.h"
+#include "sherpa-onnx/csrc/online-recognizer.h"
+#include "sherpa-onnx/csrc/online-stream.h"
+
+namespace sherpa_onnx {
+
+class OnlineRecognizerImpl {
+ public:
+  static std::unique_ptr<OnlineRecognizerImpl> Create(
+      const OnlineRecognizerConfig &config);
+
+#if __ANDROID_API__ >= 9
+  static std::unique_ptr<OnlineRecognizerImpl> Create(
+      AAssetManager *mgr, const OnlineRecognizerConfig &config);
+#endif
+
+  virtual ~OnlineRecognizerImpl() = default;
+
+  virtual void InitOnlineStream(OnlineStream *stream) const = 0;
+
+  virtual std::unique_ptr<OnlineStream> CreateStream() const = 0;
+
+  virtual std::unique_ptr<OnlineStream> CreateStream(
+      const std::vector<std::vector<int32_t>> &contexts) const {
+    SHERPA_ONNX_LOGE("Only transducer models support contextual biasing.");
+    exit(-1);
+  }
+
+  virtual bool IsReady(OnlineStream *s) const = 0;
+
+  virtual void DecodeStreams(OnlineStream **ss, int32_t n) const = 0;
+
+  virtual OnlineRecognizerResult GetResult(OnlineStream *s) const = 0;
+
+  virtual bool IsEndpoint(OnlineStream *s) const = 0;
+
+  virtual void Reset(OnlineStream *s) const = 0;
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_ONLINE_RECOGNIZER_IMPL_H_