[ENH] Add Beta Distribution to XGBoostLSS

[joshdunnlime]

XGBoostLSS uses pytorch backend with "concentration" nomenclature instead of skpro alpha and beta.

Reference Issues/PRs

No reference as it is a very simple PR.

What does this implement/fix? Explain your changes.

This adds the Beta Distribution to XGBoostLSS. It is a simple conversion from the pytorch Beta to skpro Beta. It is the simple mapping of concentration1 -> alpha and concentration0 -> beta.

Does your contribution introduce a new dependency? If yes, which one?

No.

What should a reviewer concentrate their feedback on?

Only one line of code and one line of docs changed.

Did you add any tests for the change?

Yes. Added Beta to the XGBoostLSS get_test_params.

Any other comments?

No.

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[fkiraly]

was this added only recently to xgboostlss?

[joshdunnlime]

@fkiraly Looks like it was 2 years ago: StatMixedML/XGBoostLSS@64c4b63

[fkiraly]

hm - I suppose it could have been that skpro did not have the Beta distribution yet when we added xgboostlss? Not sure, but either way thanks for completing this.