Fix imagenet when importing a dataset with non-directory files

[wonjuleee]

Summary

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) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[sooahleex]

Looks good to me.

[yunchu]

#1178

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (8f5ceb9) 79.96% compared to head (1eb656b) 79.96%.

Additional details and impacted files

@@                Coverage Diff                 @@
##           releases/1.5.0    #1179      +/-   ##
==================================================
- Coverage           79.96%   79.96%   -0.01%     
==================================================
  Files                 265      265              
  Lines               29702    29704       +2     
  Branches             5831     5831              
==================================================
+ Hits                23751    23752       +1     
- Misses               4618     4619       +1     
  Partials             1333     1333              

Flag	Coverage Δ
macos-11_Python-3.8	79.08% <33.33%> (-0.01%)	⬇️
ubuntu-20.04_Python-3.8	79.96% <33.33%> (-0.01%)	⬇️
windows-2022_Python-3.8	79.93% <33.33%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/datumaro/plugins/data_formats/imagenet.py	83.67% <33.33%> (-0.71%)	⬇️

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