Begin to support CTC models (#119)

Please see https://k2-fsa.github.io/sherpa/onnx/pretrained_models/offline-ctc/nemo/index.html for a list of pre-trained CTC models from NeMo.

sherpa-onnx/csrc/offline-ctc-model.h

@@ -0,0 +1,59 @@
+// sherpa-onnx/csrc/offline-ctc-model.h
+//
+// Copyright (c)  2022-2023  Xiaomi Corporation
+#ifndef SHERPA_ONNX_CSRC_OFFLINE_CTC_MODEL_H_
+#define SHERPA_ONNX_CSRC_OFFLINE_CTC_MODEL_H_
+
+#include <memory>
+#include <string>
+#include <utility>
+
+#include "onnxruntime_cxx_api.h"  // NOLINT
+#include "sherpa-onnx/csrc/offline-model-config.h"
+
+namespace sherpa_onnx {
+
+class OfflineCtcModel {
+ public:
+  virtual ~OfflineCtcModel() = default;
+  static std::unique_ptr<OfflineCtcModel> Create(
+      const OfflineModelConfig &config);
+
+  /** Run the forward method of the model.
+   *
+   * @param features  A tensor of shape (N, T, C). It is changed in-place.
+   * @param features_length  A 1-D tensor of shape (N,) containing number of
+   *                         valid frames in `features` before padding.
+   *                         Its dtype is int64_t.
+   *
+   * @return Return a pair containing:
+   *  - log_probs: A 3-D tensor of shape (N, T', vocab_size).
+   *  - log_probs_length A 1-D tensor of shape (N,). Its dtype is int64_t
+   */
+  virtual std::pair<Ort::Value, Ort::Value> Forward(
+      Ort::Value features, Ort::Value features_length) = 0;
+
+  /** Return the vocabulary size of the model
+   */
+  virtual int32_t VocabSize() const = 0;
+
+  /** SubsamplingFactor of the model
+   *
+   * For Citrinet, the subsampling factor is usually 4.
+   * For Conformer CTC, the subsampling factor is usually 8.
+   */
+  virtual int32_t SubsamplingFactor() const = 0;
+
+  /** Return an allocator for allocating memory
+   */
+  virtual OrtAllocator *Allocator() const = 0;
+
+  /** For some models, e.g., those from NeMo, they require some preprocessing
+   * for the features.
+   */
+  virtual std::string FeatureNormalizationMethod() const { return {}; }
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_OFFLINE_CTC_MODEL_H_