PDP variance feature importance

[RobertSamoilescu]

This PR implements the method described here to compute feature importance and feature interactions.

The main idea for the computation of both measures is to analyse the variance of the partial dependence plots, thus the implementation relies on the PartialDependece explainer.

It also adds plotting functionalities for the feature importance and feature interactions. Furthermore, since it is recommended to analyse the results in tandem with the partial dependence plots, the explanation object is designed to be compatible with the pd_plot functionality.

[codecov[bot]]

Codecov Report

Merging #758 (ca3067b) into master (313c760) will decrease coverage by 0.21%. The diff coverage is 41.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #758      +/-   ##
==========================================
- Coverage   75.72%   75.51%   -0.22%     
==========================================
  Files          72       73       +1     
  Lines        8223     8477     +254     
==========================================
+ Hits         6227     6401     +174     
- Misses       1996     2076      +80     

Flag	Coverage Δ
macos-latest-3.10	75.47% <41.50%> (?)
macos-latest-3.10.6	?
ubuntu-latest-3.10	75.47% <41.50%> (?)
ubuntu-latest-3.10.6	?
ubuntu-latest-3.7	75.20% <41.26%> (-0.33%)	:arrow_down:
ubuntu-latest-3.8	75.25% <41.26%> (-0.24%)	:arrow_down:
ubuntu-latest-3.9	75.34% <41.26%> (-0.24%)	:arrow_down:
windows-latest-3.9	73.41% <41.26%> (-0.18%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
alibi/explainers/pd_variance.py	40.72% <40.72%> (ø)
alibi/api/defaults.py	100.00% <100.00%> (ø)
alibi/explainers/__init__.py	100.00% <100.00%> (ø)
alibi/utils/missing_optional_dependency.py	94.44% <0.00%> (+0.15%)	:arrow_up:
alibi/explainers/partial_dependence.py	48.16% <0.00%> (+0.21%)	:arrow_up:
alibi/models/tensorflow/autoencoder.py	100.00% <0.00%> (+43.47%)	:arrow_up:
alibi/models/tensorflow/cfrl_models.py	100.00% <0.00%> (+75.67%)	:arrow_up:

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