Fix typings in ImageNet format

[itrushkin]

Fixes https://github.com/openvinotoolkit/datumaro/actions/runs/9870800186/job/27258510825.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (releases/1.8.0@b2b8533). Learn more about missing BASE report.

Additional details and impacted files

@@                Coverage Diff                @@
##             releases/1.8.0    #1563   +/-   ##
=================================================
  Coverage                  ?   81.04%           
=================================================
  Files                     ?      276           
  Lines                     ?    32408           
  Branches                  ?     6590           
=================================================
  Hits                      ?    26265           
  Misses                    ?     4691           
  Partials                  ?     1452           

Flag	Coverage Δ
ubuntu-20.04_Python-3.10	81.02% <100.00%> (?)
windows-2022_Python-3.10	81.02% <100.00%> (?)

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.