[DDP][BE][Docs] Clarify checkpoint support (#68827)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68827
Add a note about current checkpoint support with DDP. Note that this
does not include the features enabled with _set_static_graph yet, as it is an
undocumented private API. Once we support static graph as beta feature in OSS
we can add to the note here.
ghstack-source-id: 144285041
Test Plan: CI
Reviewed By: pritamdamania87
Differential Revision: D32624957
fbshipit-source-id: e21d156a1c4744b6e2a807b5b5289ed26701886f
