[optim] implement AdafactorSchedule (#12123)
* implement AdafactorSchedule
* typo
* fix
* Update src/transformers/optimization.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
