[FSDP] summon offload to CPU (#73904)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73904
Implement ability to offload full params to CPU in summon_full_params.
ghstack-source-id: 151311236
Test Plan: CI
Reviewed By: fegin
Differential Revision: D34707801
fbshipit-source-id: efc9c568037ddfeb9ddaff45a1a0388c6bc85825
(cherry picked from commit 4f3195d26027b5661cc6d525a30b13c6dc826ba7)
