Pascal VOC mIOU should ignore `void` labels

[bermanmaxim]

Hello, At training, the void label in Pascal VOC is mapped to the background class 0. However, for evaluation purposes, it would make sense to ignore pixels corresponding void pixels in the ground-truth, as this is standard for Pascal VOC evaluation. If I'm not mistaken, this step was not done here. I opened a similar issue on this tensorflow implementation.

[Hutai]

@bermanmaxim I really find this problem. How do you update the mIOU code？

[IssamLaradji]

You can just zero the prediction mask at the locations where the void pixels are before computing the mIoU.