Only log total_flos at the end of training
# What does this PR do?

This PR removes the addition of `total_flos` at each (and every) log, since this kind of pollutes them, and only logs it once and for all at the end of training. Users can still define their own callbacks and do more with that value if they really want to, but from what I understood. @TevenLeScao, that value is mainly necessary at the end.

Also, now that it's not in the metrics anymore, I've reverted the default compute metrics to its previous behavior (sum of all metrics) since it's the documented behavior. (cc @madlag) If we want to really change it, we need to put more examples out there.