added warning for None dataloader (#1745)

* added warning for None dataloader

* fixed variable style

* updated warning message

* remove unused import

Co-authored-by: ybrovman <ybrovman@ebay.com>

pytorch_lightning/trainer/data_loading.py

@@ -218,6 +218,9 @@ class TrainerDataLoadingMixin(ABC):
                     f'Your {mode}_dataloader has shuffle=True, it is best practice to turn'
                     ' this off for validation and test dataloaders.')
 
+        if any([dl is None for dl in dataloaders]):
+            rank_zero_warn("One of given dataloaders is None and it will be skipped.")
+
         # add samplers
         dataloaders = [self.auto_add_sampler(dl, train=False) for dl in dataloaders if dl is not None]