The class number is 18, why set `BiSeNet(19)`

[piwawa]

明明只有18个类别，为啥train.py和test.py都设置成下面这样：

n_classes = 19
net = BiSeNet(n_classes=n_classes)

另外为啥把HQ数据集的background标签去掉了，加上不是正好19个嘛？？

[yakhyo]

@piwawa , in segmentation you need to add background also as a class.