Bug when Training on multi-GPU

[ghost]

when I trained train_partset.py with the command:
python train_partseg.py --multi_gpu="1, 2" --model_name='pointnet2' --batchsize=16 --epoch=130 --step_size=30 --optimizer='Adam'
the program will stop at the first iteration of progress bar, and I even can't kill the process.

While when I trained on a single GPU with:
python train_partseg.py --gpu="2" --model_name='pointnet2' --batchsize=16 --epoch=130 --step_size=30 --optimizer='Adam'
it can be run successfully.

I don't know what's the problem and can you kindly tell me how to solve it?