A little late, but maybe someone will read it. You could try removing the keyword transforms=aug. Instead try Cityscapes(...., transform=aug, target_transform=aug). Note that there are transform and transforms as two different keywords.
If that doesnt work, you could try removing aug=T.Compose(...), and instead create a class.
class CityTransform:
    def __call__(self, image, target):
        image = *your transformation*  (image)
        target = *your transformation* (target)
        return image, target

then insert it
dataset = Cityscapes('../data/', split='val', mode='fine',
                     target_type='semantic',transforms=CityTransform)