Add one-way partial dependence and partial dependence plots

[angela97lin]

Closes #985

scikit-learn's partial_dependence does some checking to ensure that a fitted pipeline has been passed in. Unfortunately, the code used also checks for certain attributes to ensure that a classifier / regressor is passed in. There are two options:

1. Limit the available pipelines to those available in scikit-learn (not catboost or our baseline)
2. Update our _classes attribute to classes_ instead to match scikit-learn, then set _estimator_type for the input pipeline to something (and somehow revert it after? Tried looking into context managers but not sure if that's the right approach since we're creating a new attribute, not temporarily updating a pre-existing one).

For now, I've updated this PR for option 1, since the idea of creating a new attribute and temporarily setting it feels weird, but this really limits what we can pass into partial_dependence, especially when it seems like it works fine (tested briefly with catboost) once we pass all of the checks. We also can't wrap the inputted pipeline with our own pipeline attribute since a fitted pipeline is required.

[codecov]

Codecov Report

Merging #1079 into main will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1079   +/-   ##
=======================================
  Coverage   99.89%   99.89%           
=======================================
  Files         192      192           
  Lines       10894    10981   +87     
=======================================
+ Hits        10883    10970   +87     
  Misses         11       11           

Impacted Files	Coverage Δ
evalml/model_understanding/__init__.py	100.00% <ø> (ø)
evalml/model_understanding/graphs.py	100.00% <100.00%> (ø)
...lml/tests/model_understanding_tests/test_graphs.py	100.00% <100.00%> (ø)

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[dsherry]

@angela97lin looks good!

I have two change requests before merging:

• The graph is backwards! The axes need to be flipped. Noticed because the user guide example is not a function
• I found a way to get catboost working

[freddyaboulton]

@angela97lin Looks good to me! I have a minor comment on the user guide that is not blocking.