update QuickTour docs to reflect model output object
Currently, the [Quick tour](https://huggingface.co/transformers/quicktour.html#) docs shows model output as tuples when you print them out.  In the current version of 🤗, the user sees an object that inherits from the `ModelOutput` class.  Yes, you can still access this object as a tuple, but this might be confusing for many readers, especially since this is the very first document that many people see when using 🤗 .

This PR does the following things:

1. Changes code examples in the Quick Tour to show the output object, not the tuple.  
2. Minor modification in the `Model Output` doc as _both_ PyTorch and Tensorflow models return an object that is an instance of a subclass of `ModelOutput`.
 
@sgugger

P.S. I am planning to go through all of the documentation very carefully like this, please let me know if there is anything along these lines that I can pay more attention to that is needed. 

I might need some help or tips with figuring out why styling check CI is failing.  I tried to debug but it is not clear to me what is wrong<|||||>Thanks, @sgugger that worked<|||||>Thank *you* for the fixes :-)