Resuming training from a checkpoint on Windows does not resume at the correct global_step
### Who can help
Trainer: @sgugger


## Information

I've noticed that resuming training from a checkpoint on Windows fails to start at the right global step (though learning rate, loss, etc continue from where the training left off). The same doesn't happen under macOS.

## Solution?

I think the issue is in Trainer.train, specifically this line:
`self.global_step = int(model_path.split("-")[-1].split("/")[0])`

And it seems to work as expected by swapping `"/"` with `os.sep`


Seems wrong indeed. Would you mind fixing with a PR?<|||||>I'm embarrassed to say I'm not super familiar with git or open source collaboration in general, so even though it seems super trivial, I'm worried I'll mess something up. <|||||>I can fix this but it'll be next week as I'm off tonight and tying a few other loose ends.<|||||>Thank you! I'm in no personal rush – I got my aborted training to resume. I just wanted someone on the project to know about the problem/solution I found. 