Understanding The Vertical and Horizontal stack in conditional gated Pixelcnn paper
I also had the same problem when reading the paper, I believe it is not explicitly mentioned.
I think the whole image is padded with zeros on the top row when you are using the vertical stack. In this way, you ensure that the output of any pixel of row i will depend only on the pixels at row i-1 till row 0. But at the same time, you have the advantage of depth of field compositionality and remove the blind spot (given you are stacking enough convolutional layers).