COCO: when saving, don't drop standalone skeletons that have group IDs

[SpecLad]

Summary

Previously, the find_instance_anns calls would exhaust the group iterator, thus even if the condition was true, the following code would find no skeletons.

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2022 CVAT.ai Corporation
#
# SPDX-License-Identifier: MIT