[WIP][JIT] Add ScriptModule._reconstruct (#39979)
Summary:
**Summary**
This commit adds an instance method `_reconstruct` that permits users
to reconstruct a `ScriptModule` from a given C++ `Module` instance.
**Testing**
This commit adds a unit test for `_reconstruct`.
**Fixes**
This pull request fixes https://github.com/pytorch/pytorch/issues/33912.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39979
Differential Revision: D22172323
Pulled By: SplitInfinity
fbshipit-source-id: 9aa6551c422a5a324b822a09cd8d7c660f99ca5c
