To implement a multiple node sets example

[SidneyLann]

This repo is very useful for me, could you please implement a multiple node sets example. Thanks!

[joshcarty]

Thanks, that's great to hear the repository has been useful to you.

I created this example in the absence of examples in tensorflow/gnn. There are now official examples so I won't be maintaining or extending this repository.

I hope you understand. Thanks!