Save huggingface checkpoint as artifact in mlflow callback
# What does this PR do?

1. Store model checkpoints including tokenizers that are needed to reload the model from mlflow as artifacts
2. Allow model to be register-able. (they are not if log_artifacts is used to log the model)

Fixes # (issue)
https://github.com/huggingface/transformers/issues/15495
https://github.com/huggingface/transformers/issues/10881
https://github.com/huggingface/transformers/issues/7698

## 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/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] 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/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@sgugger 

_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks again!<|||||>Hi there! @swethmandava Thanks for adding this functionality. Quick question: because the artifact logging was removed, wouldn't the intermediate checkpoints not be tracked? Only the latest checkpoint would be logged as a model, right?<|||||>> Hi there! @swethmandava Thanks for adding this functionality. Quick question: because the artifact logging was removed, wouldn't the intermediate checkpoints not be tracked? Only the latest checkpoint would be logged as a model, right?

It should now save all the checkpoints. every time on_save is called