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/python/csrc/online-model-config.h

@@ -0,0 +1,16 @@
+// sherpa-onnx/python/csrc/online-model-config.h
+//
+// Copyright (c)  2023 by manyeyes
+
+#ifndef SHERPA_ONNX_PYTHON_CSRC_ONLINE_MODEL_CONFIG_H_
+#define SHERPA_ONNX_PYTHON_CSRC_ONLINE_MODEL_CONFIG_H_
+
+#include "sherpa-onnx/python/csrc/sherpa-onnx.h"
+
+namespace sherpa_onnx {
+
+void PybindOnlineModelConfig(py::module *m);
+
+}
+
+#endif  // SHERPA_ONNX_PYTHON_CSRC_ONLINE_MODEL_CONFIG_H_