def to_chw(im, order=(2, 0, 1)):
    """
    Transpose the input image order. The image layout is HWC format
    opened by cv2 or PIL. Transpose the input image to CHW layout
    according the order (2,0,1).

    Example usage:
    
    .. code-block:: python
        im = load_image('cat.jpg')
        im = resize_short(im, 256)
        im = to_chw(im)
    
    :param im: the input image with HWC layout.
    :type im: ndarray
    :param order: the transposed order.
    :type order: tuple|list 
    """
    assert len(im.shape) == len(order)
    im = im.transpose(order)
    return im