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-model-config.h

@@ -0,0 +1,48 @@
+// sherpa-onnx/csrc/online-model-config.h
+//
+// Copyright (c)  2023  Xiaomi Corporation
+#ifndef SHERPA_ONNX_CSRC_ONLINE_MODEL_CONFIG_H_
+#define SHERPA_ONNX_CSRC_ONLINE_MODEL_CONFIG_H_
+
+#include <string>
+
+#include "sherpa-onnx/csrc/online-transducer-model-config.h"
+
+namespace sherpa_onnx {
+
+struct OnlineModelConfig {
+  OnlineTransducerModelConfig transducer;
+  std::string tokens;
+  int32_t num_threads = 1;
+  bool debug = false;
+  std::string provider = "cpu";
+
+  // Valid values:
+  //  - conformer, conformer transducer from icefall
+  //  - lstm, lstm transducer from icefall
+  //  - zipformer, zipformer transducer from icefall
+  //  - zipformer2, zipformer2 transducer from icefall
+  //
+  // All other values are invalid and lead to loading the model twice.
+  std::string model_type;
+
+  OnlineModelConfig() = default;
+  OnlineModelConfig(const OnlineTransducerModelConfig &transducer,
+                    const std::string &tokens, int32_t num_threads, bool debug,
+                    const std::string &provider, const std::string &model_type)
+      : transducer(transducer),
+        tokens(tokens),
+        num_threads(num_threads),
+        debug(debug),
+        provider(provider),
+        model_type(model_type) {}
+
+  void Register(ParseOptions *po);
+  bool Validate() const;
+
+  std::string ToString() const;
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_ONLINE_MODEL_CONFIG_H_