Add metric computation of mean prediction

[gaugup]

Description

Add metric computation of mean prediction which is the mean of all prediction (regression scenario). UI seems to have "Mean prediction".

Checklist

• I have added screenshots above for all UI changes.
• I have added e2e tests for all UI changes.
• Documentation was updated if it was needed.

[codecov-commenter]

Codecov Report

Merging #1693 (fc35575) into main (6a8612b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1693   +/-   ##
=======================================
  Coverage   88.90%   88.91%           
=======================================
  Files         105      105           
  Lines        5553     5557    +4     
=======================================
+ Hits         4937     4941    +4     
  Misses        616      616           

Flag	Coverage Δ
unittests	88.91% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
erroranalysis/erroranalysis/_internal/constants.py	100.00% <100.00%> (ø)
erroranalysis/erroranalysis/_internal/metrics.py	90.24% <100.00%> (+0.77%)	⬆️

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[github-actions]

https://responsibleai.blob.core.windows.net/pullrequest/microsoft/responsible-ai-toolbox/gaugup/AddMeanPredictionMetric/dashboard/index.html

[github-actions]

https://responsibleai.blob.core.windows.net/pullrequest/microsoft/responsible-ai-toolbox/gaugup/AddMeanPredictionMetric/dashboard/index.html