[2/n] Consolidate `replicate` and `DDP`: split `forward` function (#96658)
Split `forward` function into `pre_forward` and `post_forward`, so that they can be reused in the composable API of `replicate`.
Differential Revision: [D44377456](https://our.internmc.facebook.com/intern/diff/D44377456)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96658
Approved by: https://github.com/rohan-varma
