You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: BatchNorm.m
    path of file: ./repos/cnn-for-image-retrieval/matconvnet-1.0-beta17/matlab/+dagnn
    
    the code of the file until where you have to start completion: classdef BatchNorm < dagnn.ElementWise
  methods
    function outputs = forw
    
function [derInputs, derParams] = backward(obj, inputs, params, derOutputs)
      [derInputs{1}, derParams{1}, derParams{2}, derParams{3}] = ...
        vl_nnbnorm(inputs{1}, params{1}, params{2}, derOutputs{1}) ;
      % multiply the moments update by the number of images in the batch
      % this is required to make the update additive for subbatches
      % and will eventually be normalized away
      derParams{3} = derParams{3} * size(inputs{1},4) ;
    end