Add label groups for hierarchical classification in ImageNet

[itrushkin]

Summary

This PR adds grouping of labels (directories for the ImageNet case) by their groups (parent directories).
For example, for the following folder structure

.
├── label_1
│   └── label_1_1
│       └── 1.jpg
└── label_2
    └── label_2_1
        └── 2.jpg

label groups will be label_1 and label_2.

Note: for the higher depth of nesting, names of groups will be relative paths of second-to-last directories. For the following case:

.
├── label_1
│   └── label_1_1
│       └── label_1_1_1
│           └── 1.jpg
│        
└── label_2
    └── label_2_1
        └── label_2_1_1
            └── 2.jpg

label groups will be label_1/label_1_1 and label_2/label_2_1.

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

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.22%. Comparing base (ff5fd94) to head (3f48944).
Report is 21 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/datumaro/plugins/data_formats/imagenet.py	66.66%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1645      +/-   ##
===========================================
+ Coverage    81.06%   81.22%   +0.16%     
===========================================
  Files          278      281       +3     
  Lines        32517    32889     +372     
  Branches      6607     5291    -1316     
===========================================
+ Hits         26360    26715     +355     
- Misses        4701     4723      +22     
+ Partials      1456     1451       -5     

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ubuntu-20.04_Python-3.10	81.20% <66.66%> (+0.15%)	⬆️
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[wonjuleee]

Thanks @itrushkin, could you add parent information for each label?
For instance,
LabelCategories.Category(name='Clover', parent='', attributes=set()), LabelCategories.Category(name='Clover/Ace', parent='', attributes=set())
should be turned into
LabelCategories.Category(name='Clover', parent='', attributes=set()), LabelCategories.Category(name='Clover/Ace', parent='Clover', attributes=set())

[itrushkin]

@wonjuleee Parent category property is implemented in daf6669

[wonjuleee]

LGTM! Thank you for your efforts!