compute iou

[chensonglu]

For consistency, I checked the official Pascal VOC Matlab code and ssd.pytorch https://github.com/amdegroot/ssd.pytorch. I think "+1" in lines 99, 109, and 110 of "utils.py" is different from the official implementation. In my view, your calculation is more reasonable, so this issue is just a reminder to other coders.