feat: Implements erosion & dilation in PyTorch & TF

[fg-mindee]

The use of cv2 for erosion & dilation is limiting for export in ONNX or SavedModel. Following cv2 documentation, this PR introduces the following modifications:

• adds implementation of erosion & dilation in Pytorch & TensorFlow
• adds unittests accordingly

This is the first to improve export compatibility and GPU post-processing for text detection models.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #669 (f4ad238) into main (7244d43) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #669      +/-   ##
==========================================
- Coverage   96.15%   96.15%   -0.01%     
==========================================
  Files         114      117       +3     
  Lines        4453     4472      +19     
==========================================
+ Hits         4282     4300      +18     
- Misses        171      172       +1     

Flag	Coverage Δ
unittests	96.15% <100.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
doctr/models/detection/_utils/__init__.py	100.00% <100.00%> (ø)
doctr/models/detection/_utils/pytorch.py	100.00% <100.00%> (ø)
doctr/models/detection/_utils/tensorflow.py	100.00% <100.00%> (ø)
...dels/detection/differentiable_binarization/base.py	91.30% <0.00%> (-0.63%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7244d43...f4ad238. Read the comment docs.

[charlesmindee]

Thanks for that!