derivative of the link function wrt mu

        Parameters
        ----------
        mu : array-like of legth n
        dist : Distribution instance

        Returns
        -------
        grad : np.array of length n

def gradient(self, mu, dist):
        """
        derivative of the link function wrt mu

        Parameters
        ----------
        mu : array-like of legth n
        dist : Distribution instance

        Returns
        -------
        grad : np.array of length n
        """
        return dist.levels/(mu*(dist.levels - mu))