Using IterableDatasets without __len__ for Training
Calling fit(model, trainloader, evalloader) internally calls enforce_datamodule_dataloader_override. This function
has the if statement if (train_dataloader or val_dataloaders) and datamodule:. 
  
    
      pytorch-lightning/pytorch_lightning/trainer/configuration_validator.py
    
    
         Line 13
      in
      2c935d0
    
  
  
    

        
          
           if (train_dataloader or val_dataloaders) and datamodule: 
        
    
  


This is similar to the PR #1560, the problem is that the if(dl) translates to if(bool(dl)), but there's no dataloader.bool so bool() uses dataloader.len > 0. But... dataloader.len uses IterableDataset.len for IterableDatasets for which len is undefined.
The fix is also the same, the if dl should be replaced by if dl is not None.
I will open a PR fixing this.