[docs] Add NCCL environment variable docs (#8345)
[docs] Add NCCL environment variable docs (#8345)

* Add nccl env variable docs

* Wording

* Update docs/source/guides/speed.rst

Co-authored-by: Kaushik B <2e15db51523f41103c81a3cc55419f1e40e69a68@users.noreply.github.com>

Co-authored-by: Kaushik B <2e15db51523f41103c81a3cc55419f1e40e69a68@users.noreply.github.com>
williamFalcon/pytorch-lightning,williamFalcon/pytorch-lightning