[PyTorch] Bypass dispatch for narrow() calls within split_with_sizes (#83213)
This can add up to a lot of dispatcher overhead if there are a lot of splits. split_with_sizes already has an autograd formula so this should Just Work?
Differential Revision: [D38600576](https://our.internmc.facebook.com/intern/diff/D38600576/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83213
Approved by: https://github.com/albanD
