docstr: Initialize explainer as a `shap.TreeExplainer`. 
code: def _init_shap_object(self) -> shap.TreeExplainer:
      """Initialize explainer as a `shap.TreeExplainer`."""
      return shap.TreeExplainer(
          self.predictor,
          data=self.background_data,
          model_output=self.model_output,
          feature_perturbation=self.perturbation,
      )
  
imports: import copy
  import logging
  from functools import partial
  from typing import TYPE_CHECKING, Any, Callable, Dict, List, Optional, Sequence, Tuple, Union
  import numpy
  import pandas
  import shap
  import shap.utils._legacy
  from scipy import sparse
  from scipy.special import expit
  from shap import KernelExplainer
  from alibi.api.defaults import DEFAULT_DATA_KERNEL_SHAP, DEFAULT_DATA_TREE_SHAP, DEFAULT_META_KERNEL_SHAP, DEFAULT_META_TREE_SHAP
  from alibi.api.interfaces import Explainer, Explanation, FitMixin
  from alibi.utils.wrappers import methdispatch
label_desc: Use image node_id 3 for calling the TreeShap obj's underlying member method code with example usage: obj._init_shap_object() and returns: shap