// Use the built-in operator.
  grad_outputs->push_back(
      internal::RsqrtGrad(scope, op.output(0), grad_inputs[0]));
  return scope.status();
}
REGISTER_GRADIENT_OP("Rsqrt", RsqrtGrad);

Status ExpGrad(const Scope& scope, const Operation& op,
               const std::vector<Output>& grad_inputs,
               std::vector<Output>* grad_outputs) {
  // dy/dx = exp(x) = y
  // grad(x) = grad(y) * conj(dy/dx)