[FSDP] Use unflatten_parameter in _summon_full_parameters (#72467)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72467
`_unflatten_params_as_views` does not (and cannot) delete `flat_param`. Using `_unflatten_params_as_views` to implement `_summon_full_parameters` will make the traverse of the full parameter failed -- will get the redundent flat_param.
ghstack-source-id: 148959167
Test Plan: CI
Reviewed By: rohan-varma
Differential Revision: D33989893
fbshipit-source-id: 698c97766266be01d5b567b5d5f3b2fdbf24063d
(cherry picked from commit f464c6f3909e408807594907896fd66fe639e3cb)
