question: Why is the default implementation for train_dataloader in DataHooks a warning?
context: The default implementation for these is logging a warning that nothing is implemented. 
    
      
        pytorch-lightning/pytorch_lightning/core/hooks.py
      
      
           Line 529
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             rank_zero_warn("`train_dataloader` must be implemented to be used with the Lightning Trainer") 
          
      
    
  
  
  Why isn’t the default implementation to raise a NotImplementedError? This would make errors much clearer in case users forget to override these hooks
answer: I believe this is just legacy code. No real reason. It's in the original implementation (to bolts!)
  PyTorchLightning/lightning-bolts@797464c
  Feel free to try changing it :)