is called.
`source` is the image of the source image and `target` is the image of
the target image which are both in CHW format. `flow` represents optical
flow from the source to the target whose shape is :math:`(3, H, W)`.
:math:`H` and :math:`W` are the height and the width of images.
Args:
data_dir (string): Path to the root of the training data. If this is