Commit the last step on world_process_zero in WandbCallback
# What does this PR do?

Only commit the last step on the first process (`is_world_process_zero == True`), to avoid calling `wandb.log()` without a prior call to `wandb.init()` (the latter being only called on the first process) with DDP.

Fixes (at least partially) #9623 

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests), Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@borisdayma @sgugger 

Would it make sense to move [those 2 lines](https://github.com/huggingface/transformers/blob/0dd939bf1e01594eadf21b40e5cdb07001233cbf/src/transformers/integrations.py#L573-L574) to the init method instead of setting `self._log_model` to False?<|||||>This should be equivalent indeed, and probably easier to read<|||||>Looks great!