// If `dx` is nullptr, the implementation will use dx of `OnesLike` for all
// shapes in `y`.
// The partial derivatives are returned in `dy`. `dy` should be allocated to
// size `nx`.
// `prefix` names the scope into which all gradients operations are being added.
// `prefix` must be unique within the provided graph otherwise this operation
// will fail. If `prefix` is nullptr, the default prefixing behaviour takes