@Feywell it seems like you are facing the same issue. In my case NCCL 2.7.6 resolved the issue and I was able to train my models. Infact, I had almost similar system settings:
Ubuntu 18.04
2080ti
CUDA 10.2
pytorch-nightly 1.7
python 3.7
Are you using pytorch-nightly or just pytorch? If you are not using pytorch-nightly, please try using that.