Refactor offline recognizer. (#94)

* Refactor offline recognizer.

The purpose is to make it easier to support different types of models.

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

@@ -0,0 +1,29 @@
+// sherpa-onnx/csrc/offline-recognizer-impl.h
+//
+// Copyright (c)  2023  Xiaomi Corporation
+
+#ifndef SHERPA_ONNX_CSRC_OFFLINE_RECOGNIZER_IMPL_H_
+#define SHERPA_ONNX_CSRC_OFFLINE_RECOGNIZER_IMPL_H_
+
+#include <memory>
+
+#include "sherpa-onnx/csrc/offline-recognizer.h"
+#include "sherpa-onnx/csrc/offline-stream.h"
+
+namespace sherpa_onnx {
+
+class OfflineRecognizerImpl {
+ public:
+  static std::unique_ptr<OfflineRecognizerImpl> Create(
+      const OfflineRecognizerConfig &config);
+
+  virtual ~OfflineRecognizerImpl() = default;
+
+  virtual std::unique_ptr<OfflineStream> CreateStream() const = 0;
+
+  virtual void DecodeStreams(OfflineStream **ss, int32_t n) const = 0;
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_OFFLINE_RECOGNIZER_IMPL_H_