2.  Write the op gradient with the following naming scheme:

    ```
    Status OpNameGrad(const Scope& scope, const Operation& op,
                      const std::vector<Output>& grad_inputs,
                      std::vector<Output>* grad_outputs) {
      ...
      return scope.status();
    }
    REGISTER_GRADIENT_OP("OpName", OpNameGrad);
    ```

3.  Ops gradients are implemented by using the