Fix either::map naming (#33904)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33904
This was misnamed and should actually be either::fold.
ghstack-source-id: 102050883
Test Plan: it's just a rename
Differential Revision: D20148263
fbshipit-source-id: 5d2ed92230e20e8bb7dec26ac3f26de7f03a6e39
