Polish code

paddle/fluid/framework/op_desc.cc

@@ -275,8 +275,9 @@ Attribute OpDesc::GetAttr(const std::string &name) const {
   return it->second;
 }
 
-const proto::OpProto::Attr &OpDesc::GetProtoAttr(const std::string &name) {
-  proto::OpProto &proto = OpInfoMap::Instance().Get(Type()).Proto();
+const proto::OpProto::Attr &OpDesc::GetProtoAttr(
+    const std::string &name) const {
+  const proto::OpProto &proto = OpInfoMap::Instance().Get(Type()).Proto();
   for (int i = 0; i != proto.attrs_size(); ++i) {
     const proto::OpProto::Attr &attr = proto.attrs(i);
     if (attr.name() == name) {