add setitem to class types (#25750)
Summary:
Follow up to https://github.com/pytorch/pytorch/pull/25664, add `class_type[ind] = val`. Like `__getitem__`, `__setitem__` has a custom compilation path so it wasn't added with the rest of the magic methods.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25750
Differential Revision: D17428725
Pulled By: eellison
fbshipit-source-id: ff3767ef41515baf04b0c0f5c896dbd3f1d20cd3
