[Feat] Add layout masking

[felixdittrich92]

This PR:

• Add ignore_regions to allow masking layout model detected regions before further proc
• Add cor tests

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.65%. Comparing base (369f0aa) to head (ac43fa7).

Files with missing lines	Patch %	Lines
doctr/models/predictor/base.py	86.66%	2 Missing ⚠️
doctr/models/_utils.py	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2109      +/-   ##
==========================================
- Coverage   96.65%   96.65%   -0.01%     
==========================================
  Files         166      166              
  Lines        8707     8750      +43     
==========================================
+ Hits         8416     8457      +41     
- Misses        291      293       +2     

Flag	Coverage Δ
unittests	96.65% <94.44%> (-0.01%)	⬇️

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