Calculates the negative log-likelihood of the Normal model for a minibatch

        Parameters
        ----------
        beta : np.ndarray
            Contains untransformed starting values for latent variables

        mini_batch : int
            Size of each mini batch of data

        Returns
        ----------
        The negative logliklihood of the model

def normal_mb_neg_loglik(self, beta, mini_batch):
        """ Calculates the negative log-likelihood of the Normal model for a minibatch

        Parameters
        ----------
        beta : np.ndarray
            Contains untransformed starting values for latent variables

        mini_batch : int
            Size of each mini batch of data

        Returns
        ----------
        The negative logliklihood of the model
        """     

        mu, Y = self._mb_model(beta, mini_batch)
        return -np.sum(ss.norm.logpdf(Y, loc=mu, scale=self.latent_variables.z_list[-1].prior.transform(beta[-1])))