message: Fixed static reference issue with _version_converter
diff: namespace ONNX_NAMESPACE { namespace version_conversion {
  -namespace {
  - static DefaultVersionConverter _version_converter;
  -}
  -
  ModelProto ConvertVersion(
  const ModelProto& mp_in,
  const int target_version) {
  @@ -18,7 +14,8 @@ ModelProto ConvertVersion(
  }
  }
  OpSetID target_struct = OpSetID(target_version);
  - return _version_converter.convert_version(mp_in, initial_struct, target_struct);
  + DefaultVersionConverter v;
  + return v.convert_version(mp_in, initial_struct, target_struct);
  }
  ModelProto DefaultVersionConverter::convert_version(
  