Example multiple metric

[rabsr]

Closes #981
As per #985

1. Adding example showing how to calculate multiple metrics per run
2. Fix to allow users to calculate custom metric too.

[codecov]

Codecov Report

Merging #1045 (1dcdffe) into development (da7d765) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development    #1045      +/-   ##
===============================================
- Coverage        85.50%   85.48%   -0.03%     
===============================================
  Files              127      127              
  Lines            10176    10175       -1     
===============================================
- Hits              8701     8698       -3     
- Misses            1475     1477       +2     

Impacted Files	Coverage Δ
autosklearn/evaluation/abstract_evaluator.py	88.80% <100.00%> (+0.35%)	⬆️
...eline/components/feature_preprocessing/fast_ica.py	91.30% <0.00%> (-6.53%)	⬇️
...ipeline/components/regression/gradient_boosting.py	89.42% <0.00%> (-2.89%)	⬇️
...ine/components/classification/gradient_boosting.py	91.30% <0.00%> (-0.87%)	⬇️
autosklearn/util/backend.py	76.15% <0.00%> (+1.42%)	⬆️

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[mfeurer]

Thanks a lot for your contribution. It seems that the documentation building failed due to sphinx-doc/sphinx#8616. Sphinx released a new version 20 hours ago so I restarted the tests to see if this makes the documentation building work.

[mfeurer]

Great, the new sphinx version fixed the documentation building. Will merge this now, thanks a lot!