Exporting a centroid-only model?

[jkbhagatio]

I believe that currently there is no way to export a centroid-only model via the SLEAP API or CLI. E.g., trying to using sleap-export yields the following error:

ValueError: Ground truth data was not detected... Please load both models when predicting on non-ground-truth data.

There are cases where we find it useful to have a centroid-only model, and @bruno-f-cruz has this workaround for using keras directly to perform this export: https://github.com/bruno-f-cruz/Sleap_export-Bonsai/blob/main/sleap_scripts/export_networks.ipynb (see the cell that begins with the comment "Export a centroid model")

It would be nice to have the ability to do this directly via sleap-export. Is there a reason this isn't supported? If not, maybe this Discussion can be converted into an Issue?