Randomize one of the channels of the input image

def _rgb_randomize(x, channel:int=None, thresh:float=0.3):
    "Randomize one of the channels of the input image"
    if channel is None: channel = np.random.randint(0, x.shape[0] - 1)
    x[channel] = torch.rand(x.shape[1:]) * np.random.uniform(0, thresh)
    return x