// gradient.
// If `grad` is null and `func` is in `g`, TF_GraphCopyFunction is a noop.
// `grad` must have appropriate signature as described in the doc of
// GradientDef in tensorflow/core/framework/function.proto.
//
// If successful, status is set to OK and `func` and `grad` are added to `g`.
// Otherwise, status is set to the encountered error and `g` is unmodified.