[JIT] Add reference semantics to TorchScript classes (#44324)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44324
**Summary**
This commit adds reference semantics to TorchScript class types;
modifications made to them within TorchScript will be visible in Python.
**Test Plan**
This commit adds a unit test to `TestClassType` that checks that
modifications made to a class type instance passed into TorchScript are
visible in Python after executing the scripted function or module.
**Fixes**
This commit closes #41421.
Test Plan: Imported from OSS
Reviewed By: gmagogsfm
Differential Revision: D24912807
Pulled By: SplitInfinity
fbshipit-source-id: d64ac6211012425b040b987e3358253016e84ca0
