[Fix] imgur5k test

[felixdittrich92]

This PR fixes the failing imgur5k test:

• inconsistent number of boxes samples yields to different numbers of samples in the test
• now each sample has the same number of boxes to avoid random numbers of samples (recognition test)

[codecov]

Codecov Report

Merging #903 (debf4f4) into main (56b914c) will increase coverage by 0.02%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##             main     #903      +/-   ##
==========================================
+ Coverage   94.70%   94.73%   +0.02%     
==========================================
  Files         133      134       +1     
  Lines        5442     5465      +23     
==========================================
+ Hits         5154     5177      +23     
  Misses        288      288              

Flag	Coverage Δ
unittests	94.73% <95.65%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
doctr/datasets/mjsynth.py	95.45% <95.45%> (ø)
doctr/datasets/__init__.py	100.00% <100.00%> (ø)
doctr/transforms/modules/base.py	94.59% <0.00%> (ø)
doctr/transforms/functional/base.py	97.10% <0.00%> (+1.44%)	⬆️

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[felixdittrich92]

@charlesmindee

[charlesmindee]

Thanks for the fix!