[FSDP] Break up `_post_backward_hook` into smaller funcs (#106068)
The post-backward hook has some complexity due to the different paths: {no communication hook, communication hook} x {`NO_SHARD`, `FULL_SHARD`/`SHARD_GRAD_OP`, `HYBRID_SHARD`/`_HYBRID_SHARD_ZERO2`} plus some options like CPU offloading and `use_orig_params=True` (requiring using sharded gradient views).
The PR following this one that adds async all-reduce for HSDP further complicates this since the bottom-half after all-reduce must still be run in the separate all-reduce stream, making it more unwieldy to unify with the existing bottom-half.
Nonetheless, this PR breaks up the post-backward hook into smaller logical functions to hopefully help readability.
Differential Revision: [D47852461](https://our.internmc.facebook.com/intern/diff/D47852461)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106068
Approved by: https://github.com/ezyang, https://github.com/fegin
