I assume you have downloaded gtFine_trainvaltest.zip and leftImg8bit_trainvaltest.zip from https://www.cityscapes-dataset.com/downloads/ 4 and extract to ~/BigDatas/cityscapes which has gtFine and leftImage8bit.
Reproduce steps:
git clone https://github.com/fyu/drn.git
cd drn
python3 datasets/cityscapes/prepare_data.py ~/BigDatas/cityscapes/gtFine
cp datasets/cityscapes/create_lists.sh ~/BigDatas/cityscapes
cp datasets/cityscapes/info.json ~/BigDatas/cityscapes
cd ~/BigDatas/cityscapes
sh create_lists.sh
chmod u+x segment.py

Then CUDA_VISIBLE_DEVICES=0,1 python3 segment.py train -d ~/BigDatas/cityscapes -c 19 -s 896 --arch drn_d_22 --batch-size 32 --epochs 250 --lr 0.01 --momentum 0.9 --step 100 works very well. However, CUDA_VISIBLE_DEVICES=0,1 CUDA_LAUNCH_BLOCKING=1 python3 segment.py train -d ~/BigDatas/cityscapes -c 19 -s 896 --arch drn_d_22 --batch-size 32 --epochs 250 --lr 0.01 --momentum 0.9 --step 100 will freeze like below:

2018-07-04-105307_1917x1053_scrot.jpg1917×1053 991 KB

I am doubting that I am the first man who meet the problem of “train with CUDA_LAUNCH_BLOCKING=1 will freeze” in the world, I am helpless~
Ubuntu 16.04, four GTX 1080, PyTorch 0.4 which is installeld from pip install torch.