feat: Added FP16 support for doctr.models

[fg-mindee]

Following up on #263, this PR introduces the following modifications:

• made dtype dynamic whenever possible
• added error message for unknown dtype in preprocessors
• added FP16 support to all models
• updated unittests

Any feedback is welcome!

[codecov]

Codecov Report

Merging #382 (61ddf5d) into main (58a3f8d) will decrease coverage by 0.08%.
The diff coverage is 85.24%.

@@            Coverage Diff             @@
##             main     #382      +/-   ##
==========================================
- Coverage   96.22%   96.14%   -0.09%     
==========================================
  Files          83       83              
  Lines        3554     3577      +23     
==========================================
+ Hits         3420     3439      +19     
- Misses        134      138       +4     

Flag	Coverage Δ
unittests	96.14% <85.24%> (-0.09%)	⬇️

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

Impacted Files	Coverage Δ
doctr/models/core.py	94.11% <ø> (ø)
doctr/models/recognition/transformer/tensorflow.py	88.11% <66.66%> (ø)
doctr/models/_utils.py	93.10% <75.00%> (ø)
doctr/models/detection/linknet/base.py	88.31% <75.00%> (ø)
doctr/models/preprocessor/pytorch.py	96.49% <81.81%> (-3.51%)	⬇️
doctr/models/preprocessor/tensorflow.py	96.36% <81.81%> (-3.64%)	⬇️
...dels/detection/differentiable_binarization/base.py	91.35% <88.88%> (+0.05%)	⬆️
doctr/models/detection/core.py	98.24% <100.00%> (ø)
...etection/differentiable_binarization/tensorflow.py	97.11% <100.00%> (ø)
doctr/models/detection/linknet/pytorch.py	96.90% <100.00%> (ø)
... and 7 more

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[charlesmindee]

Thanks!