[nnc] Do not fuse unsqueeze with variable dim (#58346)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58346
If `dim` is a variable, NNC doesn't know how to translate the result,
since the shape is unknown. This issue manifested as a `bad_variant_access`
when we try to pull an int constant out of that arg.
Note that, while the PE will pick up the resultant shape, it won't set guards accordingly.
ghstack-source-id: 129078971
Test Plan: new fuser test
Reviewed By: navahgar
Differential Revision: D28460956
fbshipit-source-id: 57ef918ef309ee57bfdf86717b910b6549750454
