// returns, won't affect the copy of `func`/`grad` in `g`.
// If `func` or `grad` are already in `g`, TF_GraphCopyFunction has no
// effect on them, but can establish the function->gradient relationship
// between them if `func` does not already have a gradient. If `func` already
// has a gradient different from `grad`, an error is returned.
//
// `func` must not be null.