I think the problem is that you need to clamp the values in the image
try this:
def save_im(tensor, title):
    image = tensor.cpu().clone()
    x = image.clamp(0, 255)
    x = x.view(x.size(0), 3, 480, 640)
    save_image(x, "image_{}.png".format(title))

make sure you unsqueeze the matrix to have an extra dimension or change line 3 in above function