fix: unexpected list reference issue

[eamonn-zh]

This is a serious bug, label_to_segs[label].extend(segs) unexpectedly modifies the list in object_id_to_segs[object_id] as segs are assigned to both label_to_segs[label] and object_id_to_segs[object_id] by reference. This results in some segments being assigned the wrong object id.

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