[PyTorch][easy] Add cbegin/cend to SmallVector (#64682)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64682
Looks like it was forked from llvm before cbegin and cend existed.
ghstack-source-id: 138036981
Test Plan: CI
Reviewed By: dhruvbird
Differential Revision: D30814434
fbshipit-source-id: 9740fa8d3df1c90b77298a95ab9f1d0cf8c90320
