Fix override for __iter__ (#54702)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54702
This fixes subclassing for __iter__ so that it returns an iterator over
subclasses properly instead of Tensor.
Test Plan: Imported from OSS
Reviewed By: H-Huang
Differential Revision: D27352563
Pulled By: ezyang
fbshipit-source-id: 4c195a86c8f2931a6276dc07b1e74ee72002107c
