Add holdout scores to pipeline results

[christopherbunn]

Pull Request Description

(replace this text with your description)

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After creating the pull request: in order to pass the release_notes_updated check you will need to update the "Future Release" section of docs/source/release_notes.rst to include this pull request by adding :pr:123.

[codecov[bot]]

Codecov Report

Merging #3663 (b084266) into main (5ec5e6a) will decrease coverage by 16.3%. The diff coverage is 66.7%.

@@           Coverage Diff            @@
##            main   #3663      +/-   ##
========================================
- Coverage   99.7%   83.5%   -16.2%     
========================================
  Files        335     335              
  Lines      33879   33880       +1     
========================================
- Hits       33746   28258    -5488     
- Misses       133    5622    +5489     

Impacted Files	Coverage Δ
evalml/automl/engine/engine_base.py	94.7% <50.0%> (-2.0%)	:arrow_down:
evalml/automl/automl_search.py	66.7% <100.0%> (-32.6%)	:arrow_down:
evalml/tests/automl_tests/test_search.py	0.0% <0.0%> (-100.0%)	:arrow_down:
evalml/tests/tuner_tests/test_skopt_tuner.py	0.0% <0.0%> (-100.0%)	:arrow_down:
evalml/tests/automl_tests/test_engine_base.py	0.0% <0.0%> (-100.0%)	:arrow_down:
evalml/tests/automl_tests/test_automl_utils.py	0.0% <0.0%> (-100.0%)	:arrow_down:
evalml/tests/automl_tests/test_search_iterative.py	0.0% <0.0%> (-100.0%)	:arrow_down:
evalml/tests/tuner_tests/test_grid_search_tuner.py	0.0% <0.0%> (-100.0%)	:arrow_down:
...valml/tests/automl_tests/test_default_algorithm.py	0.0% <0.0%> (-100.0%)	:arrow_down:
...valml/tests/automl_tests/test_time_series_split.py	0.0% <0.0%> (-100.0%)	:arrow_down:
... and 25 more

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