Make shap an extra dependency

[jklaise]

This PR is meant to (temporarily) make shap an optional dependency. The reason is because the build process of shap currently is not optimal and will result in RuntimeError when packages have a different runtime dependency on numpy than the build dependency of numpy used for shap.

Concretely, shap on linux is built with the latest available version of numpy (currently 1.20), whilst tensorflow has a numpy pin of 1.19.5. This means that pip install alibi results in a broken installation as shap is still built against 1.20. As far as I've looked, it is not possible to specify a different version of numpy to build shap against in a "one-pass" install of alibi (e.g. if numpy is not present, shap will be built against 1.20 regardless, if numpy is present it will be built against that version).

As a result, this PR proposes installing alibi with shap support in two steps:

pip install alibi
pip install alibi[shap]

I have added the above instructions to the README as well as at the top of the shap-specific notebooks.

This workaround should be resolved once shap specifies oldest-supported-numpy as a build dependency instead. For more information see here: shap/shap#1802

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[codecov]

Codecov Report

Merging #376 (f07d6a6) into master (d9be65f) will decrease coverage by 0.14%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #376      +/-   ##
==========================================
- Coverage   87.73%   87.59%   -0.15%     
==========================================
  Files          56       56              
  Lines        7372     7376       +4     
==========================================
- Hits         6468     6461       -7     
- Misses        904      915      +11     

Impacted Files	Coverage Δ
alibi/explainers/__init__.py	85.71% <60.00%> (-14.29%)	⬇️
alibi/explainers/anchor_base.py	89.24% <0.00%> (-2.85%)	⬇️

[RafalSkolasinski]

Looks okay but what if someone tries to import something from shap-dependent parts of library without shap being installed?
alibi/explainers/shap_wrappers.py ?

I think it will fail in quite an ugly way -> maybe worth to have some logic that do try ... except in shap imports that in case shap is not present raise a valid warning / information?

[jklaise]

Looks okay but what if someone tries to import something from shap-dependent parts of library without shap being installed?
alibi/explainers/shap_wrappers.py ?

I think it will fail in quite an ugly way -> maybe worth to have some logic that do try ... except in shap imports that in case shap is not present raise a valid warning / information?

Good point, forgot to handle this completely as it would fail immediately on alibi import since it imports the shap specific explainers too... The try ... except logic should be put in alibi.explainers.__init__ so only the relevant explainers are loaded.