Bug in Readme: Hierarchical Shrinkage Doesn't Support HistGradientBoostingRegressor

[bridgream]

The readme page claims that hierarchical shrinkage supports any sklearn tree-based model, but in reality it only works with sklearn.ensemble.GradientBoostingRegressor and sklearn.ensemble.GradientBoostingClassifier. When used with sklearn.ensemble.HistGradientBoostingRegressor, the _shrink method is nullified because neither of the two conditions is true:
https://github.com/csinva/imodels/blob/master/imodels/tree/hierarchical_shrinkage.py#L125-L127 . This is same for HistGradientBoostingClassifier.

We should either add support to hist boosting trees, or clarify this in the readme.

[csinva]

Thanks for pointing this out! I've just updated this in the readme and we'll try and support HistGradientBoostingRegressor soon!