refactor(embed): construct the embed model

[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 52.57732% with 46 lines in your changes missing coverage. Please review.

Project coverage is 57.30%. Comparing base (1b72e5e) to head (4bfec84).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/retrievals/models/embedding_auto.py	63.63%	16 Missing ⚠️
src/retrievals/models/base.py	36.36%	14 Missing ⚠️
src/retrievals/losses/base.py	12.50%	7 Missing ⚠️
src/retrievals/data/dataset.py	45.45%	6 Missing ⚠️
src/retrievals/trainer/trainer.py	50.00%	2 Missing ⚠️
src/retrievals/models/rerank.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
+ Coverage   56.54%   57.30%   +0.76%     
==========================================
  Files          39       39              
  Lines        2773     2743      -30     
==========================================
+ Hits         1568     1572       +4     
+ Misses       1205     1171      -34     

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