Avoid decomposing `_unsafe_index` in Inductor (#107882)
`_unsafe_index` was previously added to the core ATen decomp table in https://github.com/pytorch/pytorch/pull/106814, but this has performance ramifications for Inductor. Therefore, this diff removes it from the decomposition table used by Inductor.
Differential Revision: [D48649210](https://our.internmc.facebook.com/intern/diff/D48649210/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107882
Approved by: https://github.com/SherlockNoMad
