Preserve dummy channel dim.

def preserve_channel_dim(func):
    """Preserve dummy channel dim."""
    @wraps(func)
    def wrapped_function(img, *args, **kwargs):
        shape = img.shape
        result = func(img, *args, **kwargs)
        if len(shape) == 3 and shape[-1] == 1 and len(result.shape) == 2:
            result = np.expand_dims(result, axis=-1)
        return result

    return wrapped_function