feat: Added high-level predictors

[fg-mindee]

This PR introduces the following modifications:

• created DetectionPredictor that takes a list of images as inputs and returns a list of tensors, where each tensor represents all the detected bounding boxes in the image
• fixed edge case of cropping when no bounding box was passed
• created RecognitionPredictor that takes a list of image crops as inputs and returns a list of character sequences for each crop
• created OCRPredictor that takes a list of lists of images as inputs, and returns the localization + recognition results in a structured hierarchical output
• updated all unittests accordingly

Please note that for now, the OCRPredictor returns a structured output but does not use the elements defined in the module doct.documents

Successfully merging this PR will close #3 and close #4

Any feedback is welcome!

[codecov]

Codecov Report

Merging #39 (7d2f85c) into main (73b43dd) will decrease coverage by 0.11%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
- Coverage   97.86%   97.75%   -0.12%     
==========================================
  Files          19       20       +1     
  Lines         469      534      +65     
==========================================
+ Hits          459      522      +63     
- Misses         10       12       +2     

Impacted Files	Coverage Δ
doctr/models/recognition/__init__.py	100.00% <ø> (ø)
doctr/models/recognition/crnn.py	100.00% <ø> (ø)
doctr/models/core.py	94.44% <94.44%> (ø)
doctr/models/recognition/core.py	92.59% <94.73%> (+2.59%)	⬆️
doctr/models/__init__.py	100.00% <100.00%> (ø)
doctr/models/_utils.py	92.85% <100.00%> (ø)
doctr/models/detection/core.py	93.33% <100.00%> (+4.44%)	⬆️
...tr/models/detection/differentiable_binarization.py	98.18% <100.00%> (-0.05%)	⬇️
doctr/models/preprocessor.py	100.00% <100.00%> (+3.44%)	⬆️
doctr/models/recognition/postprocessor.py	94.59% <100.00%> (+0.15%)	⬆️
... and 1 more

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