Remove warning about building from source to use the NCCL backend (#34051)
Summary:
I think this warning isn't true anymore, and the NCCL backend works without PyTorch needing to be built from source.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34051
Differential Revision: D20195310
Pulled By: ezyang
fbshipit-source-id: 14f879a8c43ea5efdbdf0f638792ea2b90011f4a
