Message passing aggregation arbitrary function

[diegocervera]

Currently the MessagePassing class has three different aggregation schemes: 'add', 'mean' or 'max'. Would it be possible to use an arbitrary function to aggregate the messages, e.g. a GRU?

[wsad1]

You could override the aggregate function as done in PNAConv or GENConv.

[rusty1s]

Useful relevant discussion: #3197