Implemented non-named version of unflatten (#42563)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42563
Moved logic for non-named unflatten from python nn module to aten/native to be reused by the nn module later. Fixed some inconsistencies with doc and code logic.
Test Plan: Imported from OSS
Reviewed By: zou3519
Differential Revision: D23030301
Pulled By: heitorschueroff
fbshipit-source-id: 7c804ed0baa5fca960a990211b8994b3efa7c415
