'NoneType' object has no attribute 'lower'  while training on TPU
🐛 Bug
distributed_backend is not set to 'tpu' which breaks it here: line 
  
    
      pytorch-lightning/pytorch_lightning/trainer/distrib_data_parallel.py
    
    
         Line 412
      in
      2c935d0
    
  
  
    

        
          
           if self.distributed_backend.lower() not in ['ddp_spawn', 'ddp_cpu', 'tpu']: 
        
    
  


distributed_backend has to be explicitly specified in Trainer params to have it working which is misleading from the docs: https://pytorch-lightning.readthedocs.io/en/latest/tpu.html#distributed-backend-with-tpu
To Reproduce
Steps to reproduce the behavior:
https://www.kaggle.com/lezwon/lightning-mnist-tpu
Expected behavior
Should automatically set distributed_backend and train successfully.
Additional context
Related to :
#2698 (comment)
#2812 (comment)