Document a training loop for streaming dataset

[lhoestq]

I added some docs about streaming dataset. In particular I added two subsections:

• one on how to use map for preprocessing
• one on how to use a streaming dataset in a pytorch training loop

cc @patrickvonplaten @stevhliu if you have some comments
cc @Rocketknight1 later we can add the one for TF and I might need your help ^^'

[patrickvonplaten]

Looks great!

[stevhliu]

Nice! Left a few minor comments to clarify map-style/streaming datasets :)