nccl fixes (#14195)

nccl fixes (#14195)

Summary:
This has 4 changes

1) propagate USE_SYSTEM_NCCL. Previously it was ignored and cmake always did a FindPackage
2) respect SCCACHE_DISABLE in our caffe2 sccache wrapper for circleci
3) use SCCACHE_DISABLE when building nccl, because it triggers the same bug as when using CCACHE (already tracked in https://github.com/pytorch/pytorch/issues/13362). This was hidden because we weren't respecting USE_SYSTEM_NCCL, and were never building nccl ourselves in CI
4) In one particular CI configuration (caffe2, cuda 8, cudnn 7), force USE_SYSTEM_NCCL=1. Building the bundled nccl triggers a bug in nvlink. I've done some investigation, but this looks like a tricky, preexisting bug, so rather than hold up this diff I'm tracking it separately in https://github.com/pytorch/pytorch/issues/14486
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14195

Differential Revision: D13237502

Pulled By: anderspapitto

fbshipit-source-id: 1100ac1269c7cd39e2e0b3ba12a56a3ce8977c55