style: Updated flake8 config to spot unused imports & undefined variables

[fg-mindee]

This PR introduces the following modifications:

• updates flake8 config to spot unused imports, docstring backslack issues, undefined variables
• updated codebase to match those constraints

Any feedback is welcome!

[codecov]

Codecov Report

Merging #623 (9090fc9) into main (400aec0) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #623      +/-   ##
==========================================
+ Coverage   96.16%   96.18%   +0.02%     
==========================================
  Files         111      111              
  Lines        4300     4300              
==========================================
+ Hits         4135     4136       +1     
+ Misses        165      164       -1     

Flag	Coverage Δ
unittests	96.18% <100.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
doctr/utils/metrics.py	96.59% <ø> (ø)
doctr/datasets/datasets/base.py	95.45% <100.00%> (ø)
...dels/detection/differentiable_binarization/base.py	91.82% <0.00%> (+0.62%)	⬆️

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[charlesmindee]

thanks!