[JIT] Add support for del to TS classes (#44352)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44352
**Summary**
This commit adds support for `del` with class instances. If a class
implements `__delitem__`, then `del class_instance[key]` is syntactic
sugar for `class_instance.__delitem__[key]`.
**Test Plan**
This commit adds a unit test to TestClassTypes to test this feature.
Test Plan: Imported from OSS
Reviewed By: eellison
Differential Revision: D23603102
Pulled By: SplitInfinity
fbshipit-source-id: 28ad26ddc9a693a58a6c48a0e853a1c7cf5c9fd6
