Add missing evaluate_expr for slice_scatter, slight refactor (#105714)
The substantive change is adding slice_scatter to use evaluate_expr
(and I add a test for it).
While I'm at it, I do some cleanup: provide sizevars.evaluate_expr
directly, and rewrite all sites to use it consistently.
Fixes https://github.com/pytorch/pytorch/issues/105524
Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105714
Approved by: https://github.com/Skylion007
