Improve tqdm progress bar
At the moment the progress bar is initialized with the arg leave=False: 
  
    
      pytorch-lightning/pytorch_lightning/trainer/trainer.py
    
    
         Line 861
      in
      deffbab
    
  
  
    

        
          
           eval_results = self.evaluate(model, self.get_val_dataloaders(), 
        
    
  


Sometimes, it's nice to be able to see the previous progress bar to look at the evolution of the loss and metrics.
Would that be possible to add an arg to the trainer to be able to override default tqdm parameters?
Also, another point: tqdm progress bars can be nested (https://github.com/tqdm/tqdm#nested-progress-bars). Could we imagine having a global progress bar and then a nested progress bar for each epoch loop?