[RLlib] Fix metrics bug (wrt individual agent returns) in MultiAgentEnvRunner. (#45543)

Author: sven1977

rllib/env/multi_agent_env_runner.py

@@ -610,8 +610,9 @@ def get_metrics(self) -> ResultDict:
                     episode_return += return_eps2
                     episode_duration_s += eps2.get_duration_s()
                     for sa_eps in eps2.agent_episodes.values():
-                        agent_episode_returns[str(sa_eps.agent_id)] += return_eps2
-                        module_episode_returns[sa_eps.module_id] += return_eps2
+                        return_sa = sa_eps.get_return()
+                        agent_episode_returns[str(sa_eps.agent_id)] += return_sa
+                        module_episode_returns[sa_eps.module_id] += return_sa
                 del self._ongoing_episodes_for_metrics[eps.id_]
 
             self._log_episode_metrics(