Fix AutoMLSearch.train_pipelines not applying undersampling

[angela97lin]

Closes #1987

[codecov]

Codecov Report

Merging #2038 (1e54228) into main (eb008b3) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##             main    #2038     +/-   ##
=========================================
- Coverage   100.0%   100.0%   -0.0%     
=========================================
  Files         280      280             
  Lines       23047    22907    -140     
=========================================
- Hits        23038    22898    -140     
  Misses          9        9             

Impacted Files	Coverage Δ
evalml/automl/engine/sequential_engine.py	100.0% <100.0%> (ø)
...valml/tests/automl_tests/test_sequential_engine.py	100.0% <100.0%> (ø)
...lml/tests/model_understanding_tests/test_graphs.py	100.0% <0.0%> (ø)

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[chukarsten]

Looks good, nice and succinct ;)

[bchen1116]

LGTM!