Optimize code coverage tests

[Sichao25]

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.87642% with 112 lines in your changes are missing coverage. Please review.

Project coverage is 43.61%. Comparing base (72ade2b) to head (e34e003).

Files	Patch %	Lines
tests/test_pipeline.py	23.52%	39 Missing :warning:
tests/test_tl.py	86.41%	39 Missing :warning:
tests/test_pl.py	84.61%	20 Missing :warning:
tests/test_preprocess.py	88.23%	14 Missing :warning:

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #605       +/-   ##
===========================================
+ Coverage   22.84%   43.61%   +20.76%     
===========================================
  Files         163      161        -2     
  Lines       28256    28706      +450     
===========================================
+ Hits         6455    12520     +6065     
+ Misses      21801    16186     -5615     

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