check_model_configuration throws error for no val_dataloader even if val_check_percent == 0
The check_model_configuration method raises an error when val_dataloader is None even if val_check_percent is 0, due to the line below.

  
    
      pytorch-lightning/pytorch_lightning/trainer/trainer.py
    
    
         Line 1145
      in
      c09317e
    
  
  
    

        
          
           if self.is_overridden('validation_step', model): 
        
    
  


This ends up being a bit annoying, since a common use case is to have a LightningModule set up that has a validation_step, but for some part of development temporarily pass in no validation dataset. The current implementation already has a way for disabling validation, which is to set val_check_percent to 0. However, in this case this does not avoid the error in the line above.
Fix should be simple, simply changing the condition to
if self.is_overridden('validation_step', model) and self.val_percent_check != 0:

Or it might also make sense to simply accept the case where no val_dataloader is passed and skip the validation_step, showing a warning to the user. There seems to already be a related warning that says UserWarning: One of given dataloaders is None and it will be skipped..