Add CTC HLG decoding using OpenFst (#349)

sherpa-onnx/csrc/offline-ctc-fst-decoder-config.cc

@@ -0,0 +1,32 @@
+// sherpa-onnx/csrc/offline-ctc-fst-decoder-config.cc
+//
+// Copyright (c)  2023  Xiaomi Corporation
+
+#include "sherpa-onnx/csrc/offline-ctc-fst-decoder-config.h"
+
+#include <sstream>
+#include <string>
+
+namespace sherpa_onnx {
+
+std::string OfflineCtcFstDecoderConfig::ToString() const {
+  std::ostringstream os;
+
+  os << "OfflineCtcFstDecoderConfig(";
+  os << "graph=\"" << graph << "\", ";
+  os << "max_active=" << max_active << ")";
+
+  return os.str();
+}
+
+void OfflineCtcFstDecoderConfig::Register(ParseOptions *po) {
+  std::string prefix = "ctc";
+  ParseOptions p(prefix, po);
+
+  p.Register("graph", &graph, "Path to H.fst, HL.fst, or HLG.fst");
+
+  p.Register("max-active", &max_active,
+             "Decoder max active states.  Larger->slower; more accurate");
+}
+
+}  // namespace sherpa_onnx