test: Fixed TF loss unittest

[fg-mindee]

This PR fixes the tolerance threshold for the loss difference between straight & rotated bboxes

Any feedback is welcome!

[codecov]

Codecov Report

Merging #725 (b9cc876) into main (c32c1ed) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #725      +/-   ##
==========================================
- Coverage   96.23%   96.19%   -0.05%     
==========================================
  Files         125      125              
  Lines        4707     4707              
==========================================
- Hits         4530     4528       -2     
- Misses        177      179       +2     

Flag	Coverage Δ
unittests	96.19% <ø> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
...dels/detection/differentiable_binarization/base.py	91.66% <0.00%> (-2.23%)	⬇️
doctr/models/detection/linknet/base.py	89.10% <0.00%> (+1.98%)	⬆️

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[charlesmindee]

Thanks!