class_docstr: Compute 1st and 2nd moments online (to avoid storing all elements).
  
  See algorithm at: https://www.wikiwand.com/en/Algorithms_for_calculating_variance#/Online_algorithm
code: def variance(self):
      return self._M2 / (self._n - 1)
  
imports: import numpy
label_desc: Use image node_id 4 for calling the OnlineMoments obj's underlying member method code with example usage: obj.variance() and returns: unknown