.. _`MPI Sintel Flow Dataset`: http://sintel.is.tue.mpg.de/
This class returns tuple of (source, target, flow) as :meth:`get_example`
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.