Get stats not being used in parallel runner

[Warry98]

Hello,

I've noticed that the "get_stats" method of the environment isn't being logged in the parallel runner. Instead, the parallel runner only logs the info returned by the last step method of each env.

        # ↓ not used
        # Get stats back for each env 
        for parent_conn in self.parent_conns:
            parent_conn.send(("get_stats",None))

        env_stats = []
        for parent_conn in self.parent_conns:
            env_stat = parent_conn.recv()
            env_stats.append(env_stat)
        # ↑ not used
        cur_stats = self.test_stats if test_mode else self.train_stats
        cur_returns = self.test_returns if test_mode else self.train_returns
        log_prefix = "test_" if test_mode else ""
        infos = [cur_stats] + final_env_infos
        cur_stats.update({k: sum(d.get(k, 0) for d in infos) for k in set.union(*[set(d) for d in infos])})
        cur_stats["n_episodes"] = self.batch_size + cur_stats.get("n_episodes", 0)
        cur_stats["ep_length"] = sum(episode_lengths) + cur_stats.get("ep_length", 0)

        cur_returns.extend(episode_returns)

        n_test_runs = max(1, self.args.test_nepisode // self.batch_size) * self.batch_size
        if test_mode and (len(self.test_returns) == n_test_runs):
            self._log(cur_returns, cur_stats, log_prefix)
        elif self.t_env - self.log_train_stats_t >= self.args.runner_log_interval:
            self._log(cur_returns, cur_stats, log_prefix)
            if hasattr(self.mac.action_selector, "epsilon"):
                self.logger.log_stat("epsilon", self.mac.action_selector.epsilon, self.t_env)
            self.log_train_stats_t = self.t_env

        return self.batch