feat: Added option to use AMP with TF scripts

[fg-mindee]

This PR adds support of AMP for TensorFlow training scripts following this documentation: https://www.tensorflow.org/guide/mixed_precision.

Checking within the script, all modules and operations are indeed running the "mixed_float16" policy, but for some reasons, I haven't noticed any RAM saving on GPU 🤷‍♂️

Please note that some TF models still have issues with AMP (cf. tensorflow/tensorflow#53345) but this PR enables it for training generally speaking.

Closes #263

Any feedback is welcome!

[codecov]

Codecov Report

Merging #682 (5db608f) into main (d9f432d) will decrease coverage by 0.04%.
The diff coverage is 85.18%.

@@            Coverage Diff             @@
##             main     #682      +/-   ##
==========================================
- Coverage   96.34%   96.29%   -0.05%     
==========================================
  Files         117      117              
  Lines        4519     4540      +21     
==========================================
+ Hits         4354     4372      +18     
- Misses        165      168       +3     

Flag	Coverage Δ
unittests	96.29% <85.18%> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
...dels/detection/differentiable_binarization/base.py	90.00% <ø> (+0.55%)	⬆️
doctr/models/detection/linknet/base.py	87.12% <78.94%> (-1.90%)	⬇️
doctr/models/builder.py	99.12% <100.00%> (+0.01%)	⬆️
doctr/utils/geometry.py	98.00% <100.00%> (ø)

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[charlesmindee]

Thanks!