Sure, I see. the reason i used the brackets here is  my labels are 2-D. the corresponding information is in this link.   https://stackoverflow.com/questions/41924453/pytorch-how-to-use-dataloaders-for-custom-datasets 17                For now, I was just testing only one label. Anyway, I removed the brackets and tried again. the error is like this:
Screenshot from 2018-07-28 17-01-22.png800×122 16.9 KB