[Flax] Add remat (gradient checkpointing) (#17843)
* [Flax] Add remat (gradient checkpointing)
* fix variable naming in test
* flip: checkpoint using a method
* fix naming
* fix class naming
* apply PVP's suggestions from code review
* make fix-copies
* fix big-bird, electra, roberta
* cookie-cutter
* fix flax big-bird
* move test to common
