Fix flos single node
This PR fixes a bug typo where in single-node settings, flos in the Trainer would stay constant, and also updates the flo count in the trainer state at every log occasion (instead of every model-saving occasion) so that users that wish to use a flo-logging callback can access it more frequently. I feel like the first bug should have been caught by a test (at the moment few other people use trainer flos, since it's mostly for large-scale training and researchers that need or want to report flos, and neither group uses the Trainer a lot, so it's mostly the HF bigscience efforts), and I should make one.

@sgugger 
Not sure why the CI is failing, is it related to the PR? Doesn't look so to me but I may be missing something<|||||>No, the CI is failing all the time those days, don't worry about it.