Remove unneeded const from process group agent header (#38804)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38804
This is only needed in the process group agent implementation, and
removing it from the header file prevents other translation units that include
it from having this constant.
ghstack-source-id: 104666599
Test Plan: CI
Differential Revision: D21668514
fbshipit-source-id: 1c39cc98dea99518134c66dca3ca5b124a43de1b
