code: def featurewise_norm(x, mean=None, std=None, epsilon=1e-7):
      
      if mean:
          x = x - mean
      if std:
          x = x / (std + epsilon)
      return x
docs: Normalize every pixels by the same given mean and std, which are usually
      compute from all examples.
  
      Parameters
      -----------
      x : numpy.array
          An image with dimension of [row, col, channel] (default).
      mean : float
          Value for subtraction.
      std : float
          Value for division.
      epsilon : float
          A small position value for dividing standard deviation.
  
      Returns
      -------
      numpy.array
          A processed image.