Remove ProcessGroup RPC backend placeholder as part of 1.11 (#67363)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67363
ProcessGroup RPC backend is deprecated. In 1.10 it would throw an error to the user to be more user friendly. This PR now removes it completely.
cc pietern mrshenli pritamdamania87 zhaojuanmao satgera rohan-varma gqchen aazzolini osalpekar jiayisuse SciPioneer H-Huang
Test Plan: Imported from OSS
Reviewed By: bdhirsh
Differential Revision: D32138321
Pulled By: H-Huang
fbshipit-source-id: b4f700d8f1b1d46ada7b5062d3f754646571ea90
