[BE][FSDP] Retire `_get_full_detached_param()` (#80871)
The tests did not actually require that the parameters be detached, so this coalesces `_get_full_detached_param()` with `get_full_params()`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80871
Approved by: https://github.com/rohan-varma
