Returns gradient buffers on all contexts, in the same order
        as :py:meth:`values`.

def list_grad(self):
        """Returns gradient buffers on all contexts, in the same order
        as :py:meth:`values`."""
        if self._data is not None and self._grad is None:
            raise RuntimeError(
                "Cannot get gradient array for Parameter '%s' " \
                "because grad_req='null'"%(self.name))
        return self._check_and_get(self._grad, list)