[Flax] Dataset streaming example
# What does this PR do?

This PR adds an example of how to use dataset streaming.

@lhoestq - I would be super happy if you could take a look! I've left some comments that should help understand my thought process. I'd be very happy about some Feedback :-) 

The scripts runs as expected at the moment - after your feedback, I'll make it nicer (add better defaults) & add the eval coed as well :-)
The training speed of this script compared to `run_mlm_flax` is sadly reduced by ~50%. @lhoestq - I think the "batchify" function that is called before every training step is probably the reason for this. 