fix: Fixed param override of backbone

[fg-mindee]

This PR fixes the initialization when the feature extractor already has defined param values. Before this PR, those values were overridden with a param initialization, and this PR prevents this (for pretrained_backbone=True to work correctly).

cf. #591

Any feedback is welcome!

[charlesmindee]

Thanks!

[codecov]

Codecov Report

Merging #689 (206353f) into main (9c45ba6) will decrease coverage by 0.41%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #689      +/-   ##
==========================================
- Coverage   96.40%   95.99%   -0.42%     
==========================================
  Files         117      117              
  Lines        4540     4548       +8     
==========================================
- Hits         4377     4366      -11     
- Misses        163      182      +19     

Flag	Coverage Δ
unittests	95.99% <100.00%> (-0.42%)	⬇️

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Impacted Files	Coverage Δ
...s/detection/differentiable_binarization/pytorch.py	97.50% <100.00%> (-0.81%)	⬇️
doctr/models/detection/linknet/pytorch.py	98.07% <100.00%> (+0.03%)	⬆️
doctr/models/recognition/crnn/pytorch.py	94.18% <100.00%> (-5.82%)	⬇️
doctr/models/recognition/master/pytorch.py	93.47% <100.00%> (-2.11%)	⬇️
...dels/detection/differentiable_binarization/base.py	90.00% <0.00%> (-2.78%)	⬇️
doctr/models/builder.py	96.49% <0.00%> (-2.64%)	⬇️
doctr/models/detection/linknet/base.py	85.14% <0.00%> (-1.99%)	⬇️

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