Allow MultiGraph and MultiDiGraph conversion

[vakker]

Hi,

I have a a multi graph problem defined as an NX graph, but using from_networkx throws an error.

Changing this line to the following resolves this issue:

data['edge_index'] = edge_index[:2].view(2, -1)

However, to_networkx might be less trivial as it would need to check if all the edges are unique, or maybe Data could include a property is_multi (similarly to is_directed).

I patched this locally so it works with my stuff, but I'm happy to open a PR and work out the to_networkx part if needed.

[rusty1s]

This is interesting. from_networkx and to_networkx should definitely support multi graphs. Feel free to open a PR for it :)