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-model-config.cc

@@ -13,6 +13,7 @@ namespace sherpa_onnx {
 void OfflineModelConfig::Register(ParseOptions *po) {
   transducer.Register(po);
   paraformer.Register(po);
+  nemo_ctc.Register(po);
 
   po->Register("tokens", &tokens, "Path to tokens.txt");
 
@@ -38,6 +39,10 @@ bool OfflineModelConfig::Validate() const {
     return paraformer.Validate();
   }
 
+  if (!nemo_ctc.model.empty()) {
+    return nemo_ctc.Validate();
+  }
+
   return transducer.Validate();
 }
 
@@ -47,6 +52,7 @@ std::string OfflineModelConfig::ToString() const {
   os << "OfflineModelConfig(";
   os << "transducer=" << transducer.ToString() << ", ";
   os << "paraformer=" << paraformer.ToString() << ", ";
+  os << "nemo_ctc=" << nemo_ctc.ToString() << ", ";
   os << "tokens=\"" << tokens << "\", ";
   os << "num_threads=" << num_threads << ", ";
   os << "debug=" << (debug ? "True" : "False") << ")";