Video bug fix

[jihyeonyi]

Summary

Resolves 145146 and 145169 for datumaro format.

• 145146
  • When exporting a video several times, it should be saved only once to prevent the video from being overwritten.
• 145169
  • Exported video paths should be relative from videos/<subset> directory.

How to test

Tests are automatically done by tests in tests/unit/data_formats/datumaro/test_datumaro_format.py.

• Test for 145146: test_export_video_only_once
• Test for 145169: test_can_save_and_load

Checklist

• [x] I have added unit tests to cover my changes.​
• [ ] I have added integration tests to cover my changes.​
• [x] I have added the description of my changes into CHANGELOG.​
• [ ] I have updated the documentation accordingly

License

• [x] 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) 2024 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 81.81818% with 14 lines in your changes missing coverage. Please review.

Please upload report for BASE (releases/1.8.0@bd87793). Learn more about missing BASE report.

:exclamation: Current head 4caf468 differs from pull request most recent head d82e3c8

Please upload reports for the commit d82e3c8 to get more accurate results.

Files	Patch %	Lines
src/datumaro/plugins/data_formats/imagenet.py	78.72%	3 Missing and 7 partials :warning:
src/datumaro/plugins/data_formats/image_dir.py	83.33%	1 Missing and 2 partials :warning:
src/datumaro/plugins/data_formats/yolo/exporter.py	50.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## releases/1.8.0 #1547 +/- ## ================================================= Coverage ? 81.04% ================================================= Files ? 276 Lines ? 32408 Branches ? 6590 ================================================= Hits ? 26265 Misses ? 4691 Partials ? 1452 ``` | [Flag](https://app.codecov.io/gh/openvinotoolkit/datumaro/pull/1547/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openvinotoolkit) | Coverage Δ | | |---|---|---| | [ubuntu-20.04_Python-3.10](https://app.codecov.io/gh/openvinotoolkit/datumaro/pull/1547/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openvinotoolkit) | `81.02% <81.81%> (?)` | | | [windows-2022_Python-3.10](https://app.codecov.io/gh/openvinotoolkit/datumaro/pull/1547/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openvinotoolkit) | `81.02% <81.81%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openvinotoolkit#carryforward-flags-in-the-pull-request-comment) to find out more.

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