Mean absolute error (#10927)

* added mean absolute error to loss_functions.py

* added doctest to mean absolute error to loss_functions.py

* fixed long line in loss_functions.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* fixed error in MAE

* Update machine_learning/loss_functions.py

Co-authored-by: Tianyi Zheng <[email protected]>

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Tianyi Zheng <[email protected]>