message: Add "arg_attr" to FunctionDef.
diff: @@ -30,6 +30,13 @@ message FunctionDef {
  // Attributes specific to this function definition.
  map<string, AttrValue> attr = 5;
  + // Attributes for function arguments. These attributes are the same set of
  + // valid attributes as to _Arg nodes.
  + message ArgAttrs {
  + map<string, AttrValue> attr = 1;
  + }
  + map<uint32, ArgAttrs> arg_attr = 7;
  +
  // NOTE: field id 2 deleted on Jan 11, 2017, GraphDef version 21.
  reserved 2;
  