PyTorch adapter: fix loading tasks that have already been cached

[SpecLad]

Motivation and context

This was broken in 4fc494f.

Add a test to cover this case.

Fixes #6047.

How has this been tested?

Unit tests.

Checklist

• I submit my changes into the develop branch
• I have added a description of my changes into the CHANGELOG file
• [ ] I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (see GitHub docs)
• [ ] I have increased versions of npm packages if it is necessary
  (cvat-canvas,
  cvat-core,
  cvat-data and
  cvat-ui)

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.

[codecov]

Codecov Report

Merging #6396 (65999f0) into develop (b415fec) will increase coverage by 0.25%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6396      +/-   ##
===========================================
+ Coverage    80.64%   80.89%   +0.25%     
===========================================
  Files          332      332              
  Lines        38438    38438              
  Branches      6923     6923              
===========================================
+ Hits         30997    31096      +99     
+ Misses        7125     7028      -97     
+ Partials       316      314       -2     

Components	Coverage Δ
cvat-ui	75.13% <ø> (+0.46%)	⬆️
cvat-server	86.24% <100.00%> (+0.06%)	⬆️