Update model_handler.py

[waqarsqureshi]

use the OpenCV contour function to extract contour instead of scikit-image function. to solve the problem of poor drawing of the contour see the issue 4660
#4660

Motivation and context

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have added a description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

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• 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.
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# Copyright (C) 2022 Intel Corporation
#
# SPDX-License-Identifier: MIT

[waqarsqureshi]

#4660

[bsekachev]

Hi, @waqarsqureshi

Do you think we need it if CVAT can support working with masks now?
#4543

[waqarsqureshi]

@bsekachev The function that i proposed is when we use automated masking using an existing semantic segmentation. If CVAT is already using this function then it is fine.
Use the following
contours, hierarchy = cv2.findContours(mask_by_label, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE

instead
contours = find_contours(mask_by_label, 0.5)

[nmanovic]

@yasakova-anastasia , could you please look at the PR and give your feedback?

[yasakova-anastasia]

It really works better for semantic segmentation.

[waqarsqureshi]

Great! to see it resolved!

[nmanovic]

@waqarsqureshi , thanks for your contribution.