Setup c10d extension Backend class attr the same way as builtin ones (#66991)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66991
Currently, c10d extensions uses Backend.NAME to store the creator
function. However, builtin ones use that same field to store the
name. This commit makes c10d extensions comply with builtin ones,
and uses a dedicated `_plugins` field to store creator functions.
Thanks bryanmr for pointing this out.
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: D31820307
Pulled By: mrshenli
fbshipit-source-id: 259769ebfc80c0c9fc44d25498c8d19a3a09d1bc
