message: Handle fixed data types in AllowedDataTypes()
diff: @@ -818,6 +818,9 @@ DataTypeSet AllowedDataTypes(const OpDef::AttrDef& attr_def) {
  }
  DataTypeSet AllowedDataTypes(const OpDef& op_def, const TypeAttrId& t_attr_id) {
  + if (t_attr_id.attr_name.empty()) {
  + return ToSet(t_attr_id.fixed_type);
  + }
  const OpDef::AttrDef* attr_def = FindAttr(t_attr_id.attr_name, op_def);
  CHECK(attr_def);
  return AllowedDataTypes(*attr_def);
  