auto gx_2 = Identity(scope, grad_inputs[0]);
  return BinaryGradCommon(scope, op, grad_outputs, gx_1, gx_2);
}
REGISTER_GRADIENT_OP("Add", AddGrad);
REGISTER_GRADIENT_OP("AddV2", AddGrad);

Status SubGrad(const Scope& scope, const Operation& op,
               const std::vector<Output>& grad_inputs,
               std::vector<Output>* grad_outputs) {
  // y = x_1 - x_2
  // dy/dx_1 = 1
  // dy/dx_2 = -1