fix: deprecated clipping + removed 'None' metrics

[charlesmindee]

This PR fixes:

• a deprecation warning in both pytorch training scripts with gradient clipping function.
• a bug occurring when no boxes at all are found (edge case): the current LocalizationMetric gives back a None for both precision & IoU and this is blowing up the training script because metrics can't be logged properly.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #432 (b93b5c2) into main (0dded4e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #432   +/-   ##
=======================================
  Coverage   95.84%   95.84%           
=======================================
  Files          96       96           
  Lines        3948     3948           
=======================================
  Hits         3784     3784           
  Misses        164      164           

Flag	Coverage Δ
unittests	95.84% <ø> (ø)

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Impacted Files	Coverage Δ
doctr/models/recognition/crnn/pytorch.py	98.03% <0.00%> (ø)

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[fg-mindee]

Thanks for the PR!
Agree with the inplace clipping, but I added a comment for the metric

[fg-mindee]

Thanks for the edit, I just added a small suggestion for the log message 👌

[fg-mindee]

Thanks!