fix: UnboundLocalError with bar plots using dictionary input

[thatlittleboy]

Overview

Description of the changes proposed in this pull request:

• Fixes the UnboundLocalError by assigning the cohorts variable when the input is a dictionary (previously, only an assertion was checked with no binding of the cohorts variable).
• Added a test for ensuring that dictionary input to shap.plots.bar works as expected (just a basic plot).
• Added some additional tests to ensure the input type to shap.plots.bar is either an Explanation, a Cohorts, or a dictionary of Explanation objects with the same number of feature columns in each Explanation object.

Checklist

• Closes BUG: shap.plots.bar with cohorts dict throws UnboundLocalError #3000
• All pre-commit checks pass.
• Unit tests added (if fixing a bug or adding a new feature)
• Added entry to CHANGELOG.md (if changes will affect users)

[codecov]

Codecov Report

Merging #3001 (743fe68) into master (29940b4) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3001      +/-   ##
==========================================
+ Coverage   54.73%   54.79%   +0.05%     
==========================================
  Files          90       90              
  Lines       12887    12895       +8     
==========================================
+ Hits         7054     7066      +12     
+ Misses       5833     5829       -4     

Impacted Files	Coverage Δ
shap/plots/_bar.py	59.54% <100.00%> (+3.41%)	⬆️

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[connortann]

LGTM, nice one!