* checkpoint model on first step callback
* remove debug
* add test cases; update existing tests not to save on first step
* move test out of solo
* delete
* default to False
* typo
* offload activations to disk instead of CPU RAM
* add prefetch
* Disco :dance:
* include offload_disk in e2e test for AC
* document and make sure to cleanup
* fix annotation to match docs
* fix docs build
* address PR feedback
* add e2e smoke test for using activation/gradient checkpointing with offload
* disable duplicate code check for the test
* fix relative import
* seq len too small to test this dataset with packing
* Fix checkpoint ptaching for tests
