Incorrect batch size tracking in training and validation steps
🐛 Bug
Batch sizes are tracked both in training and evaluation loops to reduce the Train/Eval results on epoch end.
In both cases len(batch) is used to find the current batch_size which is incorrect, for example MNIST loader will return 2 since batch = batch_data, batch_target.
Training loop:

  
    
      pytorch-lightning/pytorch_lightning/trainer/training_loop.py
    
    
         Line 1026
      in
      b40de54
    
  
  
    

        
          
           training_step_output.track_batch_size(len(split_batch)) 
        
    
  


Evaluation loop:

  
    
      pytorch-lightning/pytorch_lightning/trainer/evaluation_loop.py
    
    
         Line 339
      in
      b40de54
    
  
  
    

        
          
           output.track_batch_size(len(batch)) 
        
    
  


Expected behavior
Match the actual batch_size