[worker] fix: create a new event loop if none exists when building rollouts (#3803)

Author: ChangyWen

verl/workers/fsdp_workers.py

@@ -630,8 +630,7 @@ def _build_rollout(self, trust_remote_code=False):
         # For sync mode, we directly switch to trainer mode here.
         # For async mode, we can't call run_until_complete here, so we will switch to trainer mode in AgentLoopManager.
         if rollout_config.mode == "sync" and self._is_actor:
-            loop = asyncio.get_event_loop()
-            loop.run_until_complete(self.trainer_mode())
+            asyncio.run(self.trainer_mode())
 
     async def rollout_mode(self):
         """Context switch hybridengine to rollout mode."""