about use multi-GPU in 2_train.py

I tried to allocate 2 GPUs to 2_train.py, But it did not work. 
How should I do?

```
import os
os.environ["CUDA_VISIBLE_DEVICES"] = '1,2,3'
```

```
 model = UnetVggCC()
 model = torch.nn.DataParallel(model)
 model.cuda()
```
