Improve performance of LeftRight::read() (#30282)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30282
The atomic increment/decrements in LeftRight::read() were measurable in perf benchmarks. Let's improve their perf.
ghstack-source-id: 94443230
Test Plan: unit tests, perf benchmarks
Differential Revision: D18650228
fbshipit-source-id: d184ce8288510ab178e7c7da73562609d1ca3c9f
