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-nemo-enc-dec-ctc-model-config.cc

@@ -0,0 +1,35 @@
+// sherpa-onnx/csrc/offline-nemo-enc-dec-ctc-model-config.cc
+//
+// Copyright (c)  2023  Xiaomi Corporation
+
+#include "sherpa-onnx/csrc/offline-nemo-enc-dec-ctc-model-config.h"
+
+#include "sherpa-onnx/csrc/file-utils.h"
+#include "sherpa-onnx/csrc/macros.h"
+
+namespace sherpa_onnx {
+
+void OfflineNemoEncDecCtcModelConfig::Register(ParseOptions *po) {
+  po->Register("nemo-ctc-model", &model,
+               "Path to model.onnx of Nemo EncDecCtcModel.");
+}
+
+bool OfflineNemoEncDecCtcModelConfig::Validate() const {
+  if (!FileExists(model)) {
+    SHERPA_ONNX_LOGE("%s does not exist", model.c_str());
+    return false;
+  }
+
+  return true;
+}
+
+std::string OfflineNemoEncDecCtcModelConfig::ToString() const {
+  std::ostringstream os;
+
+  os << "OfflineNemoEncDecCtcModelConfig(";
+  os << "model=\"" << model << "\")";
+
+  return os.str();
+}
+
+}  // namespace sherpa_onnx