Attach a gradient buffer to this NDArray, so that `backward`
        can compute gradient with respect to it.

        Parameters
        ----------
        grad_req : {'write', 'add', 'null'}
            How gradient will be accumulated.
            - 'write': gradient will be overwritten on every backward.
            - 'add': gradient will be added to existing value on every backward.
            - 'null': do not compute gradient for this NDArray.
        stype : str, optional
            The storage type of the gradient array. Defaults to the same stype of this NDArray.

def attach_grad(self, grad_req='write', stype=None):
        """Attach a gradient buffer to this NDArray, so that `backward`
        can compute gradient with respect to it.

        Parameters
        ----------
        grad_req : {'write', 'add', 'null'}
            How gradient will be accumulated.
            - 'write': gradient will be overwritten on every backward.
            - 'add': gradient will be added to existing value on every backward.
            - 'null': do not compute gradient for this NDArray.
        stype : str, optional
            The storage type of the gradient array. Defaults to the same stype of this NDArray.
        """
        from . import zeros as _zeros
        if stype is not None:
            grad = _zeros(self.shape, stype=stype)
        else:
            grad = op.zeros_like(self)  # pylint: disable=undefined-variable
        grad_req = _GRAD_REQ_MAP[grad_req]
        check_call(_LIB.MXAutogradMarkVariables(
            1, ctypes.pointer(self.handle),
            ctypes.pointer(mx_uint(grad_req)),
            ctypes.pointer(grad.handle)))