[FSDP] Make summon_full_params a public method (#73116)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73116
Users may need summon_full_params() to get the original parameters.
ghstack-source-id: 150134237
Test Plan: CI
Reviewed By: rohan-varma
Differential Revision: D34353034
fbshipit-source-id: ac69cc032da177903cd9969094f3f82dc6a61636
(cherry picked from commit 55d34fdee3778110a165a13ae987d0339e8d33c7)
