Fix an invalid call to pycocotools.mask.iou

[IRDonch]

Summary

The pyiscrowd parameter is supposed to have the same length as gt, which in this case is 1, not 2. This mistake used to be harmless, but an assert for this was added in the upstream repository (see ppwwyyxx/cocoapi@4f034008), so if a new release of pycocotools comes out, the current version will start crashing.

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) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT