[FSDP] Remove `forward_prefetch` (#84600)
We are removing the `forward_prefetch` option. By the nature of async GPU kernel execution, launching the CPU kernel for the next layer's all-gather early does not actually improve performance. Moreover, the existing `forward_prefetch` uses the post-forward order instead of the pre-forward order, which leads to mis-targeted prefetched all-gathers.
Differential Revision: [D39454217](https://our.internmc.facebook.com/intern/diff/D39454217)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84600
Approved by: https://github.com/zhaojuanmao
