Fix RMSE gradient

[NicolaCourtier]

Description

Amend the formula for the gradient for the RootMeanSquaredErrorCost.

Issue reference

Fixes #354

Review

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.33%. Comparing base (fc6f1c9) to head (046946c).
Report is 618 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #355   +/-   ##
========================================
  Coverage    97.33%   97.33%           
========================================
  Files           42       42           
  Lines         2404     2404           
========================================
  Hits          2340     2340           
  Misses          64       64           

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[BradyPlanden]

Looks good, nice catch @NicolaCourtier! Please add a changelog entry as well :)