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
docstr: Args:
      x (float or np.ndarray): must have the same shape.
code: def feed(self, x):
      """
      Args:
          x (float or np.ndarray): must have the same shape.
      """
      self._n += 1
      delta = x - self._mean
      self._mean += delta * (1.0 / self._n)
      delta2 = x - self._mean
      self._M2 += delta * delta2
  
imports: import numpy
label_desc: Use image node_id 2 for calling the OnlineMoments obj's underlying member method code with example usage: obj.feed(x) without return types