refactor: Avoids instantiating orientation predictor when unnecessary

[fg-mindee]

This PR introduces the following modifications:

• simply saves a bit of memory by avoiding to instantiating the crop orientation predictor when it will never be used.
• refactors predictor constructor (moving as many things as possible to base): that way, kwargs of DocumentBuilder can be directly passed to the high-level API ocr_predictor
• updates unittests to check that the crop orientation predictor is only there when needed

Any feedback is welcome!

[codecov]

Codecov Report

Merging #809 (0450bbf) into main (45d4bfd) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #809      +/-   ##
==========================================
- Coverage   96.01%   96.00%   -0.01%     
==========================================
  Files         131      131              
  Lines        4942     4935       -7     
==========================================
- Hits         4745     4738       -7     
  Misses        197      197              

Flag	Coverage Δ
unittests	96.00% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
doctr/models/zoo.py	100.00% <ø> (ø)
doctr/models/predictor/base.py	100.00% <100.00%> (ø)
doctr/models/predictor/pytorch.py	97.14% <100.00%> (-0.36%)	⬇️
doctr/models/predictor/tensorflow.py	100.00% <100.00%> (ø)

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[charlesmindee]

thanks!

[charlesmindee]

Thanks!