enable fast_dev_run without a validation loop (#1779)

* fix val dataloader

* Update evaluation_loop.py

pytorch_lightning/callbacks/progress.py

@@ -93,7 +93,7 @@ class ProgressBarBase(Callback):
         """
         trainer = self.trainer
         total_val_batches = 0
-        if trainer.fast_dev_run:
+        if trainer.fast_dev_run and trainer.val_dataloaders is not None:
             total_val_batches = len(trainer.val_dataloaders)
         elif not self.trainer.disable_validation:
             is_val_epoch = (trainer.current_epoch + 1) % trainer.check_val_every_n_epoch == 0

pytorch_lightning/trainer/evaluation_loop.py

@@ -360,6 +360,10 @@ class TrainerEvaluationLoopMixin(ABC):
             dataloaders = self.val_dataloaders
             max_batches = self.num_val_batches
 
+        # enable fast_dev_run without val loop
+        if dataloaders is None:
+            return
+
         # cap max batches to 1 when using fast_dev_run
         if self.fast_dev_run:
             max_batches = 1