GitLab

* Introduce saveable callbacks

* Add note

* Test for non-present and flag

* Support early stopping and refusing to train further

* Update docstring

* More saving

* Import oopsie

* Apply suggestions from code review
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

* Make it go through TrainerArguments

* Document

* Fix test

* Apply suggestions from code review
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>

* Rework to allow for duplicates

* CLean

* Fix failing tests

---------
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>