Add _unsafe_index decomp (#106814)
Summary:
Redirect `aten._unsafe_index` to `aten.index` through a decomposition.
Also add it to the list of core decompositions.
Test Plan: contbuild and OSS CI (similar to D40075277)
Differential Revision: D48163393
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106814
Approved by: https://github.com/SherlockNoMad
