Calculates gradients based on the SVRG update rule.

def _update_svrg_gradients(self):
        """Calculates gradients based on the SVRG update rule.
        """
        param_names = self._exec_group.param_names
        for ctx in range(self._ctx_len):
            for index, name in enumerate(param_names):
                g_curr_batch_reg = self._exec_group.grad_arrays[index][ctx]
                g_curr_batch_special = self._mod_aux._exec_group.grad_arrays[index][ctx]
                g_special_weight_all_batch = self._param_dict[ctx][name]
                g_svrg = self._svrg_grads_update_rule(g_curr_batch_reg, g_curr_batch_special,
                                                      g_special_weight_all_batch)
                self._exec_group.grad_arrays[index][ctx] = g_svrg