Added mean absolute error in linear regression (#7003)

* Added mean absolute error in linear regression

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Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>

* Apply suggestions from code review

* Update linear_regression.py

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>
Co-authored-by: Caeden Perelli-Harris <caedenperelliharris@gmail.com>