[export] Remove WrapperModule. (#121042)
Summary: WrapperModule seems a good idea but may introduce some surprising behavior to users, for example, it never registers enclosed modules as submodules and therefore it's unclear that's the state dict for the exported program should look like, because some people may argue to include every state in state dict but others want to keep them as constants.
Test Plan: CI
Reviewed By: tugsbayasgalan
Differential Revision: D54326331
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121042
Approved by: https://github.com/angelayi
