Update CONTRIBUTING.md to remove ProcessGroupAgent (#63160)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/63160
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D30284439
Pulled By: H-Huang
fbshipit-source-id: 53c31b6917ef5e2125e146fb0ed73ae3d76a8cf9
