fix teardown bug

Author: awaelchli

pytorch_lightning/loops/epoch/training_epoch_loop.py

@@ -227,7 +227,7 @@ def on_run_end(self) -> List[List[STEP_OUTPUT]]:
 
     def teardown(self) -> None:
         """Frees memory of tracked epoch outputs."""
-        self.epoch_output = None
+        self._epoch_output = None
 
     def _on_train_epoch_end_hook(self, processed_epoch_output: List[List[STEP_OUTPUT]]) -> None:
         """Runs ``on_train_epoch_end hook``."""