Code stuck on "initalizing ddp" when using more than one gpu 
🐛 Bug
I am trying to run a pytorch lightning model on a 4-GPU node. In my trainer, if I specify
pl.Trainer(gpus=[0])

It runs fine. However, once I add another GPU
pl.Trainer(gpus=[0,1,2,3])

I get this output:
GPU available: True, used: True
TPU available: False, using: 0 TPU cores
LOCAL_RANK: 0 - CUDA_VISIBLE_DEVICES: [0,1,2,3]
initializing ddp: GLOBAL_RANK: 0, MEMBER: 1/4
initializing ddp: GLOBAL_RANK: 1, MEMBER: 2/4
initializing ddp: GLOBAL_RANK: 2, MEMBER: 3/4
initializing ddp: GLOBAL_RANK: 3, MEMBER: 4/4
And the model just hangs there forever. I have tried this with only 2 GPUs and get the same behavior.
Any idea why this may happen? I have tried with both ddp and ddp_spawn.

PyTorch Version-- tried both 1.4 and 1.7
OS-- Linux
Installed with pip
Python version: 3.8.5
CUDA/cuDNN version: 10.1
GPU models and configuration: NVIDIA K80s