Fix datumaro keypoint loading

[jihyeonyi]

Summary

This fixes the bug in loading a keypoint dataset in datumaro format.
The visibility information will be filled correctly by this fix.

How to test

Checklist

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

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) 2024 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (releases/1.10.0@fccb17a). Learn more about missing BASE report.

Additional details and impacted files

@@                Coverage Diff                 @@
##             releases/1.10.0    #1644   +/-   ##
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  Coverage                   ?   81.11%           
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  Files                      ?      283           
  Lines                      ?    32830           
  Branches                   ?     5288           
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  Hits                       ?    26629           
  Misses                     ?     4750           
  Partials                   ?     1451           

Flag	Coverage Δ
ubuntu-20.04_Python-3.10	81.09% <ø> (?)
windows-2022_Python-3.10	81.08% <ø> (?)

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