Why hasn't the velocity field v been predicted?

[zzhang-del]

When inputting, U (u, v) is trained as two channels by the network, but in the output image, only the velocity field u is present without v. As shown in the train auto code: inputloss: dict=model. loss_fn（
Labels=labels [:,: 1], preds=inputs [:,: 1], the author only uses the output of the first channel for loss, so why do we need to input the v velocity field?