RandomSurvivalForest.apply raises AttributeError

[Vincent-Maladiere]

Describe the bug

The apply method inherited from scikit-learn is broken. After fitting, apply raises AttributeError since it is not implemented in the SurvivalTree class.

Code Sample to Reproduce the Bug

from sksurv.datasets import load_whas500
from sksurv.ensemble.forest import RandomSurvivalForest

X_sk, y_sk = load_whas500()
rf = RandomSurvivalForest()
rf.fit(X_sk, y_sk)
rf.apply(X_sk)

Expected Results
Same data-structure as returned by scikit-learn RF.

Actual Results

Traceback (most recent call last)
[/usr/local/lib/python3.7/dist-packages/sklearn/ensemble/_forest.py](https://localhost:8080/#) in <genexpr>(.0)
    251             verbose=self.verbose,
    252             **_joblib_parallel_args(prefer="threads"),
--> 253         )(delayed(tree.apply)(X, check_input=False) for tree in self.estimators_)
    254 
    255         return np.array(results).T
AttributeError: 'SurvivalTree' object has no attribute 'apply'

Versions
Please execute the following snippet and paste the output below.

System:
    python: 3.7.13 (default, Apr 24 2022, 01:04:09)  [GCC 7.5.0]
executable: /usr/bin/python3
   machine: Linux-5.4.188+-x86_64-with-Ubuntu-18.04-bionic

Python dependencies:
          pip: 21.1.3
   setuptools: 57.4.0
      sklearn: 1.0.2
        numpy: 1.21.6
        scipy: 1.7.3
       Cython: 0.29.32
       pandas: 1.3.5
   matplotlib: 3.2.2
       joblib: 1.1.0
threadpoolctl: 3.1.0

Built with OpenMP: True
sksurv: 0.17.2
cvxopt: 1.3.0
cvxpy: 1.2.1
numexpr: 2.8.3
osqp: 0.6.2

[Vincent-Maladiere]

Would you prefer me to submit a PR to fix this or do it yourself?

[sebp]

Thanks for reporting this bug. PRs are always welcome!