[FSDP] summon full params staticmethod
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75423
Make summon_full_params a static method. We still retain the old summon_full_params as a private API `_summon_full_params` and there are a couple of callsites to this within only FSDP file, but we can remove these as well.
Differential Revision: [D35444539](https://our.internmc.facebook.com/intern/diff/D35444539/)
Approved by: https://github.com/awgu
