Returns:
            mean, max: two stats of the runners, to be added to backend

def reset_stats(self):
        """
        Returns:
            mean, max: two stats of the runners, to be added to backend
        """
        scores = list(itertools.chain.from_iterable([v.total_scores for v in self._runners]))
        for v in self._runners:
            v.total_scores.clear()

        try:
            return np.mean(scores), np.max(scores)
        except Exception:
            logger.exception("Cannot compute total scores in EnvRunner.")
            return None, None