Implement scatter reductions (CUDA), remove divide/subtract (#41977)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/33394 .
This PR does two things:
1. Implement CUDA scatter reductions with revamped GPU atomic operations.
2. Remove support for divide and subtract for CPU reduction as was discussed with ngimel .
I've also updated the docs to reflect the existence of only multiply and add.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41977
Reviewed By: mruberry
Differential Revision: D23748888
Pulled By: ngimel
fbshipit-source-id: ea643c0da03c9058e433de96db02b503514c4e9c
