title: Compute Kendall's Tau for a list of list
body: <p>I created a model that can order objects of different length, and I want to measure its performance. The model receive as an input a scrambled objects and output the objects that are ordered. For example, I have as an input x= [[ 5, 6, 4, 3, 1, 2], [3,2,4,1], [2,1,3],.....] and the output is y=[[5, 6, 3, 1, 2, 4], [ 1,3, 2, 4], [1,2,3].....]
  I want to use the Kendall's tau to compute the correlation of the list x and y. But I am not sure if I can do that, because many examples in the web showing use of Kendall's Tau with only 1D list, so should I compute Kendall's Tau for every list separately and average them? and if I don't have to do them separately, how can I deal with the different length lists?
  I tried to do scipy.stat.kendalltau(x,y) but it gives me an error because of the different lengths of lists.</p>