Revert "Rename DispatchKey.PrivateUse1 to custom device in torchgen. (#99406)"
This reverts commit c0ecd9895831f9329bd189de9b1e28ad68c93b5b.
Reverted https://github.com/pytorch/pytorch/pull/99406 on behalf of https://github.com/ezyang due to we're doing it another way ([comment](https://github.com/pytorch/pytorch/pull/99406#issuecomment-1540295309))
