Raise Error when Stacked Ensemble is passed to explain_predictions

[bchen1116]

fix #1863

[codecov]

Codecov Report

Merging #2033 (04dac0f) into main (79128bc) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##             main    #2033     +/-   ##
=========================================
+ Coverage   100.0%   100.0%   +0.1%     
=========================================
  Files         278      278             
  Lines       22714    22748     +34     
=========================================
+ Hits        22705    22739     +34     
  Misses          9        9             

Impacted Files	Coverage Δ
...nderstanding/prediction_explanations/explainers.py	100.0% <100.0%> (ø)
evalml/tests/conftest.py	100.0% <100.0%> (ø)
...s/prediction_explanations_tests/test_explainers.py	100.0% <100.0%> (ø)

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[freddyaboulton]

Thank you @bchen1116 !! This looks great.

[jeremyliweishih]

LGTM - maybe adding coverage for multiclass or time series would be good but not blocking.

[angela97lin]

LGTM! Just had a comment about the fixtures; they're defined as pipelines but are actually estimators?

[chukarsten]

Nice work, friendo!