cnn.py doesn't conform to PEP8

[oswinso]

I'm getting some errors about cnn.py not conforming to PEP8 standards, ie.

transform = transforms.Compose([
                 transforms.ToTensor(),
            ])

class SegmentationModel(object): # PEP8: expected 2 blank lines, found 1

Let's make cnn.py conform to PEP8.

AC: No PEP8 errors in cnn.py.