[bugfix] fix vllm sleep&wake_up produces meaningless output (#5143)

* fix vllm meaningless output

* add comments

Author: hjh0119

swift/trainers/rlhf_trainer/grpo_trainer.py

@@ -842,6 +842,9 @@ def _fast_infer(self, inputs: InputsType) -> Tuple[InputsType, OutputsType]:
                     outputs = self._infer_single_or_multi_turn(inputs, self.request_config)
 
             if self.vllm_mode == 'colocate' and self.args.sleep_level > 0:
+                # Reset prefix cache before sleeping to prevent using stale cache upon waking up
+                # https://github.com/modelscope/ms-swift/pull/5143
+                self.engine.engine.reset_prefix_cache()
                 self.engine.engine.sleep(level=self.args.sleep_level)
                 empty_cache()