[FSDP] Skip `_use_sharded_views()` for `SHARD_GRAD_OP` (#98250)
This PR has `SHARD_GRAD_OP` (and `_HYBRID_SHARD_ZERO2`) skip `_use_sharded_views()` in the post-forward reshard since the strategy does not free the unsharded flat parameter and can preserve the unsharded views. This saves nontrivial CPU overhead both in the post-forward reshard (`_use_sharded_views()`) and the pre-backward unshard (`_use_unsharded_views()`).
<details>
<summary>(Before) Pre-backward hook: 4.356 ms</summary>
<img width="812" alt="Screenshot 2023-04-03 at 6 32 19 PM" src="https://user-images.githubusercontent.com/31054793/229641309-778cf1f9-4b5b-42ec-b2d8-0a1e6e7ce330.png">
</details>
<details>
<summary>(After) Pre-backward hook: 1.044 ms</summary>
</details>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98250
Approved by: https://github.com/rohan-varma
