List all the output NDArray.

        Returns
        -------
        A list of ndarray bound to the heads of executor.

def _get_outputs(self):
        """List all the output NDArray.

        Returns
        -------
        A list of ndarray bound to the heads of executor.
        """
        out_size = mx_uint()
        handles = ctypes.POINTER(NDArrayHandle)()
        check_call(_LIB.MXExecutorOutputs(self.handle,
                                          ctypes.byref(out_size), ctypes.byref(handles)))
        num_output = out_size.value
        outputs = [_ndarray_cls(NDArrayHandle(handles[i])) for i in range(num_output)]
        return outputs