[BE] Remove empty pre and post proc functions (#100780)
These functions are noops, so just use the collective() override.
Differential Revision: [D45632429](https://our.internmc.facebook.com/intern/diff/D45632429/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100780
Approved by: https://github.com/Skylion007, https://github.com/fegin
