Simplify code with decltype(auto) (#30922)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30922
New c++14 feature we can use now
ghstack-source-id: 103767403
Test Plan: waitforsandcastle
Differential Revision: D18869644
fbshipit-source-id: 54541c8004b2116386668a31eb9b0410a603b7dc
