feat: add estimator type with reg and clf

Author: NeroHin

ngboost/api.py

@@ -96,6 +96,8 @@ def __init__(
             validation_fraction,
             early_stopping_rounds,
         )
+        
+        self._estimator_type = "regressor"
 
     def __getstate__(self):
         state = super().__getstate__()
@@ -172,6 +174,7 @@ def __init__(
             tol,
             random_state,
         )
+        self._estimator_type = "classifier"
 
     def predict_proba(self, X, max_iter=None):
         """