refactor(ranking): clean the code

[LongxingTan]

Description

This PR ...

Checklist

• Linked issues (if existing)
• Amended changelog.md for large changes (and added myself there as contributor)
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

Thank you for joining. Have fun coding!

[codecov]

Codecov Report

Attention: Patch coverage is 65.70048% with 71 lines in your changes missing coverage. Please review.

Project coverage is 56.54%. Comparing base (56d9153) to head (115a5f4).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/retrievals/models/rerank.py	55.42%	37 Missing ⚠️
src/retrievals/models/base.py	70.37%	24 Missing ⚠️
src/retrievals/models/utils.py	75.60%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
+ Coverage   55.34%   56.54%   +1.19%     
==========================================
  Files          39       39              
  Lines        2768     2773       +5     
==========================================
+ Hits         1532     1568      +36     
+ Misses       1236     1205      -31     

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