add MSE and MAE metrics - V2
@mariosasko New PR here. I'm not sure how to add you as a co-author here.  Also I see flake8 tests are failing, any inputs on how to resolve this ?
Also, let me know if any other changes are required. Thank you. The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_3874). All of your documentation changes will be reflected on that endpoint. Great. Thank you. Thanks so much for this 🙏 💯 
Created a new pull request to resolve unrelated changes in PR caused due to rebasing.
Ref Older PR :  [#3845](https://github.com/huggingface/datasets/pull/3845) 

Feature request here : Add support for continuous metrics (RMSE, MAE) [#3608](https://github.com/huggingface/datasets/issues/3608)