[DAR-5342][External] Allow import of complex polygons in COCO

[JBWilkie]

Problem

COCO segmentations can have >1 polygon (an array of arrays containing x, y). Currently we only select the first element

Solution

Allow import of multiple segmentation array elements as a single polygon. This effectively adds support for importing complex polygons

Changelog

Allowed import of multi-array COCO segmentations

[linear]

DAR-5342 Support multi array coco segmentation imports

[dorfmanrobert]

I would think the opposite assumption would make most sense here: that users expect an instance mask to be treated as a single complex polygon rather than separate polygon annotations. Since we only hit this with is_crowd=0, my understanding is that this would mean the masks are components of the same instance. If is_crowd=1, it would make sense to import as separate polygons

Is there a reason we are assuming users want multiple polygon annotations for an instance segmentation that has disconnected components?