Returns a gradient buffer for this parameter on one context.

        Parameters
        ----------
        ctx : Context
            Desired context.

def grad(self, ctx=None):
        """Returns a gradient buffer for this parameter on one context.

        Parameters
        ----------
        ctx : Context
            Desired context.
        """
        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, ctx)