fix: Fixed mask computation of DBNet

[fg-mindee]

This PR simply fixes the segmentation mask building: I believe we have never entered that loop, because considering its shape, it's impossible that it would have worked.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #753 (d297906) into main (45e1647) will decrease coverage by 0.20%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #753      +/-   ##
==========================================
- Coverage   96.20%   95.99%   -0.21%     
==========================================
  Files         129      129              
  Lines        4799     4799              
==========================================
- Hits         4617     4607      -10     
- Misses        182      192      +10     

Flag	Coverage Δ
unittests	95.99% <0.00%> (-0.21%)	⬇️

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Impacted Files	Coverage Δ
...dels/detection/differentiable_binarization/base.py	90.76% <0.00%> (ø)
doctr/models/recognition/predictor/tensorflow.py	91.17% <0.00%> (-5.89%)	⬇️
doctr/models/_utils.py	94.50% <0.00%> (-4.40%)	⬇️
doctr/models/builder.py	96.49% <0.00%> (-2.64%)	⬇️
doctr/transforms/modules/base.py	94.44% <0.00%> (-1.39%)	⬇️

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[charlesmindee]

Thanks for that fix!