Using default path in ModelCheckpoint does not work 
According to the implementation, passing None as model path uses the default path for checkpoints. However, the following test prevents this feature to be used when save_top_k > 0:

  
    
      pytorch-lightning/pytorch_lightning/callbacks/model_checkpoint.py
    
    
         Line 89
      in
      3e8f2d9
    
  
  
    

        
          
           if save_top_k > 0 and os.path.isdir(filepath) and len(os.listdir(filepath)) > 0: 
        
    
  


It results in the following error:
st = os.stat(s)
TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType
Also, as a side issue, the example for model checkpointing from the documentation suggests using os.getcwd() as path and save_top_k=True, which results in the warning being displayed and potentially all files of current directory being removed. New users have to be very careful if they don't wand their source code deleted.