test: Improves coverage by adding missing unittests

[fg-mindee]

This PR generally improves coverage by introducing the following modifications:

• renamed doctr/models/recognition/predictor/base.py → doctr/models/recognition/predictor/_utils.py
• added unittest for PyTorch recognition predictor inference
• added unittest for file_utils
• added unittest for crop split/merge

Any feedback is welcome!

[codecov]

Codecov Report

Merging #545 (a07ba71) into main (bf327eb) will increase coverage by 0.59%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #545      +/-   ##
==========================================
+ Coverage   95.36%   95.95%   +0.59%     
==========================================
  Files         109      109              
  Lines        4182     4182              
==========================================
+ Hits         3988     4013      +25     
+ Misses        194      169      -25     

Flag	Coverage Δ
unittests	95.95% <100.00%> (+0.59%)	⬆️

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Impacted Files	Coverage Δ
doctr/models/recognition/predictor/_utils.py	100.00% <ø> (ø)
doctr/models/recognition/predictor/pytorch.py	90.90% <100.00%> (+3.03%)	⬆️
doctr/models/recognition/predictor/tensorflow.py	91.17% <100.00%> (ø)
doctr/models/predictor/pytorch.py	100.00% <0.00%> (+33.33%)	⬆️

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[charlesmindee]

Thanks!