MMD computation - square root missing

[tathagatv]

The compute_mmd function in dist_helper.py should compute square root of disc(s1, s1) + disc(s2, s2) - 2*disc(s1, s2) right? Here is a reference for the same.

[sajjadGG]

The function returns MMD^2 the same as other graph generation models like https://github.com/JiaxuanYou/graph-generation/blob/master/test_MMD.py