 .fit() hangs when using DDP with relative imports in main code
🐛 Bug
My training script is a module within a package. If my module uses relative imports and ddp backend, it throws an error about relative imports and hangs. Using ddp_spawn backend and relative imports works as expected.
The process becomes unresponsive even to Ctrl-C and I have to kill it and its subprocesses by PID.
Traceback (most recent call last):
  File "/home/shrek/pytorch-lightning-segmentation-lapa/seg_lapa/train.py", line 13, in <module>
    from .networks.deeplab.deeplab import DeepLab
ImportError: attempted relative import with no known parent package

Using absolute imports instead of relative imports fixes the issue.
Please reproduce using the BoringModel
Not sure how to reproduce on the BoringModel, since this bug involves creating a python package and importing modulest.
To Reproduce
Create a simple python package and import module using relative imports
# Relative Imports - hangs
from .networks.deeplab.deeplab import DeepLab
from .loss_func import CrossEntropy2D
from .dataloaders import LapaDataset, DatasetSplit

# Absolute Imports - works
# from seg_lapa.networks.deeplab.deeplab import DeepLab
# from seg_lapa.loss_func import CrossEntropy2D
# from seg_lapa.dataloaders import LapaDataset, DatasetSplit

trainer = pl.Trainer(gpus=[0, 1], distributed_backend="ddp", num_nodes=1, precision=32)
trainer.fit(model, train_loader)
Expected behavior
I would expect ddp to work regardless of absolute/relative imports. The model should start training per normal.
Environment
* CUDA:
	- GPU:
		- GeForce GTX 1080 Ti
		- GeForce GTX 1080 Ti
	- available:         True
	- version:           10.2
* Packages:
	- numpy:             1.19.2
	- pyTorch_debug:     False
	- pyTorch_version:   1.6.0
	- pytorch-lightning: 1.0.2
	- tqdm:              4.50.2
* System:
	- OS:                Linux
	- architecture:
		- 64bit
		- ELF
	- processor:         x86_64
	- python:            3.8.5
	- version:           #109-Ubuntu SMP Fri Jun 19 11:33:10 UTC 2020



PyTorch Version (e.g., 1.0): 1.6.0
OS (e.g., Linux): Ubuntu 18.04
How you installed PyTorch (conda, pip, source): pip install within conda env
Build command you used (if compiling from source): N/A
Python version: 3.8.5
CUDA/cuDNN version:
GPU models and configuration:
Any other relevant information:

Additional context