[FSDP] Fix summon_full_params test (#74456)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74456
This test did not actually do any CPU offloading. Adding it revealed
an issue in no_shard (currently only when world_size == 1) case which is
tracked for a fix in https://github.com/pytorch/pytorch/issues/74166
ghstack-source-id: 152057449
Test Plan: CI
Reviewed By: zhaojuanmao
Differential Revision: D35003793
fbshipit-source-id: ffb5f8ea897cda84584ae83d362bea3c0c407c3b
(cherry picked from commit 06afebaa9cb33da1907c951100ccd7a6fb113cf2)
