c10d: Don't add NCCL backend by default without CUDA (#119149)
The NCCL backend requires CUDA (including devices) to be available. So don't use that backend by default if that isn't the case to avoid the following error when creating a CPU-only device mesh:
> RuntimeError: ProcessGroupNCCL is only supported with GPUs, no GPUs found!
Fixes #117746
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119149
Approved by: https://github.com/kwen2501
