[const_fold] Fix call_module const folding (#68614)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68614
We need to copy modules over to the `split` graph during const folding. We were previously only doing so from the non-constant submod, but we need to do this for the constant one as well in case some `call_module` is const folded.
Test Plan: Added unit test
Reviewed By: wushirong, 842974287
Differential Revision: D32543289
fbshipit-source-id: 80d1d0ce2c18a665b00e1343d6c55d939390ab10
