[FSDP][1/N]Add device_mesh to FSDPstate (#102317) (#102551)
This PR creates a device_mesh and share it across all FSDP state. The device_mesh will later be used to test out dtensor state_dict (1d device_mesh).
Approved by: https://github.com/awgu
Add device mesh to fsdp state
skip dist.get_world_size(pg) != dist.get_world_size()
address test_fake_pg.py test failure
fix test_fake_py.py failure
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102551
Approved by: https://github.com/fegin
