test: Added unittest for PyTorch high-level zoo

[fg-mindee]

This PR introduces the following modifications:

• renames test_models_tf.py into test_models_zoo_tf.py
• adds unittest for PyTorch OCR zoo

Any feedback is welcome!

[charlesmindee]

Thanks!

[codecov]

Codecov Report

Merging #499 (110b8c5) into main (6517000) will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #499      +/-   ##
==========================================
+ Coverage   94.97%   95.14%   +0.16%     
==========================================
  Files         108      108              
  Lines        4181     4181              
==========================================
+ Hits         3971     3978       +7     
+ Misses        210      203       -7     

Flag	Coverage Δ
unittests	95.14% <ø> (+0.16%)	⬆️

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Impacted Files	Coverage Δ
doctr/models/builder.py	93.00% <0.00%> (-1.00%)	⬇️
...dels/detection/differentiable_binarization/base.py	91.97% <0.00%> (+0.61%)	⬆️
...s/detection/differentiable_binarization/pytorch.py	98.29% <0.00%> (+0.85%)	⬆️
doctr/models/recognition/crnn/pytorch.py	100.00% <0.00%> (+1.09%)	⬆️
doctr/models/predictor/pytorch.py	51.28% <0.00%> (+12.82%)	⬆️

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