Thanks for your response, as far as I understand I need to change “classes=11” to “classes = 12”. I have played with the “classes” variable. I set it to 9,10,11 and 12  just for curiosity. it worked with 12 and I got metrics like that.
" Epoch:1/1… Train Loss: 1.807… Val Loss: 1.497… Train mIoU:0.097… Val mIoU: 0.122… Train Acc:0.463… Val Acc:0.534… Time: 71.30m
Total time: 71.30 m "
I got one more thing to ask. As I said earlier, my mask images got 10 classes originally, but resizing operations added some black bars to them but they are not labelled. So I guess we shouldn’t treat them as a class right?