*
   * <p>Subtypes are always listed before supertypes. But the reverse is not true. A type isn't
   * necessarily a subtype of all the types following. Order between types without subtype
   * relationship is arbitrary and not guaranteed.
   *
   * <p>If this type is a type variable or wildcard, upper bounds that are themselves type variables
   * aren't included (their super interfaces and superclasses are).
   */

// 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.