// attributes. Number type attrs and DataType attrs work fine without this.
  // Consider getting rid of this and making the behavior between number types
  // and string consistent.
  forward_op_.attrs.BuildNodeDef();
  // TODO(b/170307493): Populate skip_input_indices here.
  std::unique_ptr<GradientFunction> backward_fn;
  TF_RETURN_IF_ERROR(registry_.Lookup(forward_op_, &backward_fn));