fix: linknet hyperparameters postprocessing + demo for rotation model

[charlesmindee]

This PR adjusts the Linknet parameters in the postprocessing to have better performances, and includes the linknet_resnet18_rotation model in the streamlit demo. Pages are rendered only for straight models (for now), and we don't write the words above each boxes if the model is a rotation model.
This demo will be improved later, but for the moment it allows anyone to quickly see the performances of our rotation model without modifying the behavior for all other models.

Any feedback is welcome!

This is how it renders for the demo:

[codecov]

Codecov Report

Merging #865 (bbdcce3) into main (fc8efc9) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #865   +/-   ##
=======================================
  Coverage   94.84%   94.84%           
=======================================
  Files         133      133           
  Lines        5200     5200           
=======================================
  Hits         4932     4932           
  Misses        268      268           

Flag	Coverage Δ
unittests	94.84% <100.00%> (ø)

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

Impacted Files	Coverage Δ
doctr/models/detection/linknet/base.py	88.69% <100.00%> (-0.10%)	⬇️
doctr/utils/visualization.py	89.65% <100.00%> (+0.08%)	⬆️
doctr/models/zoo.py	100.00% <0.00%> (ø)
doctr/utils/data.py	88.67% <0.00%> (ø)
doctr/datasets/svt.py	97.22% <0.00%> (ø)
doctr/datasets/cord.py	97.43% <0.00%> (ø)
doctr/datasets/ic03.py	97.36% <0.00%> (ø)
doctr/datasets/ic13.py	96.15% <0.00%> (ø)
doctr/datasets/svhn.py	94.73% <0.00%> (ø)
doctr/models/_utils.py	97.59% <0.00%> (ø)
... and 39 more

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[fg-mindee]

Thanks @charlesmindee 🙏
What about the page synthesis? Is it still working?

[fg-mindee]

Thanks for the PR! I think there are a few things that might need tweaking for the next release, but this can be done in a later PR 👍