Returns gradient buffer attached to this NDArray.

def grad(self):
        """Returns gradient buffer attached to this NDArray."""
        from . import _ndarray_cls
        hdl = NDArrayHandle()
        check_call(_LIB.MXNDArrayGetGrad(self.handle, ctypes.byref(hdl)))
        if hdl.value is None:
            return None
        return _ndarray_cls(hdl)