[FSDP] Ignore buffers that are non-persistent. (#85740)
A buffer can be registered as non-persistent. A non-persistent buffer won't be in the state_dict.
Differential Revision: [D39858689](https://our.internmc.facebook.com/intern/diff/D39858689/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85740
Approved by: https://github.com/awgu, https://github.com/rohan-varma
