question: DDP NCCL freezes in docker AWS Jupyter
context: ❓ Questions and Help
  Problem
  Jupyter terminal freezes, and connection to AWS node closes.
  The problem is reproducible with any Lightning example.
  What have you tried?
  python pl_examples/basic_examples/image_classifier.py --gpus 4 --accelerator ddp
  What's your environment?
  Linux using docker image
  Lightning 1.0.4
  pytorch 1.6
  cuda 10.2
answer: The solution was to use
  export NCCL_SOCKET_IFNAME=lo