Update distributed.rst for ProcessGroup Extensions (#71482)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71482
cc pietern mrshenli pritamdamania87 zhaojuanmao satgera rohan-varma gqchen aazzolini osalpekar jiayisuse SciPioneer H-Huang
Test Plan: Imported from OSS
Reviewed By: rohan-varma
Differential Revision: D33745986
Pulled By: mrshenli
fbshipit-source-id: fe2d0491901bf00be09deb5c556bc1e1d359b725
(cherry picked from commit be5104bfd7e15b027647d2b8bdcd7adbe5de241d)
