Add MAE logging

[frostedoyster]

Implements logging of MAEs. This is optional during training, and it defaults to False due to the already large amount of information being printed (training/validation loss/energy/force/stress etc). During evaluation, it will always be displayed, as there are way fewer things to print

Contributor (creator of pull-request) checklist

• [x] Tests updated (for new features and bugfixes)?
• [x] Documentation updated (for new features)?

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📚 Documentation preview 📚: https://metatrain--361.org.readthedocs.build/en/361/

[abmazitov]

Thank you @frostedoyster! This looks good to me and I think we can merge without hesitation