code: def fit(self, X, y):
      self._impurity_calculation = self._calculate_variance_reduction
      self._leaf_value_calculation = self._mean_of_y
      super(RegressionTree, self).fit(X, y)
  
imports: from __future__ import division, print_function
  import numpy
  from mlfromscratch.utils import divide_on_feature, train_test_split, standardize, mean_squared_error
  from mlfromscratch.utils import calculate_entropy, accuracy_score, calculate_variance
label_desc: Use image node_id 3 for calling the RegressionTree obj's underlying member method code with example usage: obj.fit(X, y) without return types