Cleanup AnchorGenerator and PostProcess

[zhiqwang]

We refactor AnchorGenerator in this PR.

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[codecov]

Codecov Report

Merging #203 (b059a01) into main (9b24cf5) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #203      +/-   ##
==========================================
+ Coverage   93.61%   93.62%   +0.01%     
==========================================
  Files          11       11              
  Lines         673      675       +2     
==========================================
+ Hits          630      632       +2     
  Misses         43       43              

Flag	Coverage Δ
unittests	93.62% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
test/test_models.py	95.47% <100.00%> (+0.08%)	⬆️
test/test_models_anchor_utils.py	100.00% <100.00%> (ø)

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