Symintify getitem and add the required helper functions (#86207)
Note that this might not cover every use of the function (we know it doesn't)
But this is enough to get few models passing.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86207
Approved by: https://github.com/ezyang, https://github.com/Chillee, https://github.com/bdhirsh
