class_docstr: A class that can be used to replace shap.TreeExplainer so that
  the wrapper can be tested without fitting a model and instantiating
  an actual explainer.
code: def __init__(self, predictor, seed=None, *args, **kwargs):
      self.seed = seed
      np.random.seed(self.seed)
      self.model = predictor
      self.n_outputs = predictor.out_dim
  
file_path: alibi/alibi/explainers/tests/utils.py
imports: import numpy
label_desc: Use image node_id 1 to create a new MockTreeExplainer object  with example: obj = MockTreeExplainer(predictor, seed)