Clarify use of label assignment

[berombau]

I can't find in this code base a way to assign instance labels like in e.g. linear_sum_assignment in hover_net:pair_coordinates or omnipose.metrics.

Just using remap_label will not be enough as the number of labels can differ or the first label will not always be the optimal assignment. In e.g. metric.functional.panoptic_quality, you don't need it always because an IoU threshold at >0.5 has only one assignment. But for <0.5 IoU, I don't see how the current code solves the assignment problem. Can you clarify this? Is it unsupported and should all instance metrics use >0.5 IoU assignment?

[okunator]

Yes, you're correct. I have not implemented the assigmnent problem for iou < 0.5 cases. For now, the instance metrics should be used with >0.5 iou assignment. I'll add support for the cases with < 0.5 iou in the next release.