[SPMD] Remove the code branches only used in SPMD mode from distributed.py (#55353)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55353
Remove all the code branches that will only be executed when `device_ids > 1`.
Some helper functions are also removed:
1. `_verify_replicas_within_process` and `verify_replicas_within_process`
2. `_replicate_modules_within_process`
3. `parallel_apply`
The next step is deprecating `_module_copies` field.
ghstack-source-id: 126201121
Test Plan: waitforbuildbot
Reviewed By: rohan-varma
Differential Revision: D27552201
fbshipit-source-id: 128d0216a202f5b1ba4279517d68c3badba92a6c
