This function returns the default sl3 learner stack used by the
high-level hte3 wrappers. The stack always includes the core learners
Lrnr_glmnet and Lrnr_gam. It adds the following optional
learners only when their supporting runtime packages are available:
Lrnr_earth: Multivariate Adaptive Regression Spline Model.Lrnr_ranger: Random Forest Model.Lrnr_xgboost_early_stopping: XGBoost models with early stopping across a small set of depth and shrinkage configurations.
If optional packages are unavailable, get_autoML() emits a warning once
and returns the available safe subset rather than failing at runtime.
Value
A sl3::Stack object containing the available safe subset of the
predefined default learner library.