[Flax MLM] Refactor run mlm with optax (#11745)
* refactor
* update
* update
* update
* refactor run mlm
* finalize
* refactor more
* fix typo
* update
* finish refactor
* modify run mlm
* Apply suggestions from code review
* Apply suggestions from code review
* Apply suggestions from code review
* small fixes
* upload
* upload
* finish run mlm script
Co-authored-by: Patrick von Platen <patrick@huggingface.co>
