feat: improve loading time when test only

[charlesmindee]

This PR moves the train set loader after the test-only check to avoid a 1-min wait when there is no need to load the full train set.

Any feedback is welcome!

[fg-mindee]

Thanks!

[codecov]

Codecov Report

Merging #294 (97b6ff9) into main (6933091) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
+ Coverage   96.07%   96.12%   +0.04%     
==========================================
  Files          48       48              
  Lines        2089     2089              
==========================================
+ Hits         2007     2008       +1     
+ Misses         82       81       -1     

Flag	Coverage Δ
unittests	96.12% <ø> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
...tr/models/detection/differentiable_binarization.py	93.65% <0.00%> (+0.39%)	⬆️

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