[loops] Reset reference to dataloader iterator on run end (#9386)

* [loops] Reset reference to dataloader iterator on run end

Author: ananthsub

CHANGELOG.md

@@ -334,6 +334,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed `replace_sampler` missing the batch size under specific conditions ([#9367](https://github.com/PyTorchLightning/pytorch-lightning/pull/9367))
 
 
+- Fixed freeing data iterators in loop `on_run_end` ([#9386](https://github.com/PyTorchLightning/pytorch-lightning/pull/9386))
+
+
 ## [1.4.5] - 2021-08-31
 
 - Fixed reduction using `self.log(sync_dict=True, reduce_fx={mean,max})` ([#9142](https://github.com/PyTorchLightning/pytorch-lightning/pull/9142))

pytorch_lightning/loops/epoch/evaluation_epoch_loop.py

@@ -135,6 +135,7 @@ def on_run_end(self) -> EPOCH_OUTPUT:
         outputs = self.outputs
         # free memory
         self.outputs = []
+        self.dataloader_iter = None
         return outputs
 
     def evaluation_step(self, batch: Any, batch_idx: int, dataloader_idx: int) -> Optional[STEP_OUTPUT]:

pytorch_lightning/loops/epoch/training_epoch_loop.py

@@ -232,6 +232,8 @@ def on_run_end(self) -> None:
         if self._num_training_batches_reached(self.is_last_batch):
             self.update_lr_schedulers("epoch", update_plateau_schedulers=True)
 
+        self.dataloader_iter = None
+
     def teardown(self) -> None:
         self._results.cpu()
         self.batch_loop.teardown()