class_docstr: Predefined generic distribution class to use for customization distribution.
  
  The class has four methods:
      - link: link function transforms the probability of response variable to a continuous scale that is unbounded
      - init: computes numerator and denominator of the initial value.
      - gradient: computes (Negative half) Gradient of deviance function at predicted value for an actual response
      - gamma: computes numerator and denominator of terminal node estimate
  
  For customization a loss function, the gradient and gamma methods are important.
  
  To customize a special type of problem we recommend you to inherit from predefined classes:
      - CustomDistributionGaussian - for regression problems
      - CustomDistributionBernoulli - for 2-class classification problems
      - CustomDistributionMultinomial - for n-class classification problems
docstr: Contribution for GBM's leaf node prediction (numerator and denominator).
  Important for customization of a loss function.
  
  :param w: weight
  :param y: actual response
  :param z: residual
  :param f: predicted response including offset
  :return: list [weighted contribution to gamma numerator, weighted contribution to gamma denominator]
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code: def gamma(self, w, y, z, f):
      """
      Contribution for GBM's leaf node prediction (numerator and denominator).
      Important for customization of a loss function.
  
      :param w: weight
      :param y: actual response
      :param z: residual
      :param f: predicted response including offset
      :return: list [weighted contribution to gamma numerator, weighted contribution to gamma denominator]
      """
      return 1
  
label_desc: Use image node_id 4 for calling the CustomDistributionGeneric obj's underlying member method code with example usage: obj.gamma(w, y, z, f) and returns: int