Fix type checker issue with explicit cast of ref_model object (#4457)

Co-authored-by: Jeff Yang <[email protected]>
(cherry picked from commit 6211fd4)

pytorch_lightning/trainer/properties.py

@@ -15,7 +15,7 @@
 import os
 from abc import ABC
 from argparse import ArgumentParser, Namespace
-from typing import List, Optional, Union, Type, TypeVar
+from typing import List, Optional, Union, Type, TypeVar, cast
 
 from pytorch_lightning.callbacks import Callback, ProgressBarBase, ModelCheckpoint
 from pytorch_lightning.core.lightning import LightningModule
@@ -154,6 +154,7 @@ def progress_bar_callback(self):
     def progress_bar_dict(self) -> dict:
         """ Read-only for progress bar metrics. """
         ref_model = self.model if not self.data_parallel else self.model.module
+        ref_model = cast(LightningModule, ref_model)
         return dict(**ref_model.get_progress_bar_dict(), **self.logger_connector.progress_bar_metrics)
 
     @property