HeteroConv does not support add_self_loop models

[Padarn]

🐛 Describe the bug

Brought up by @rusty1s in the discussion here, it was noted that when using a HetreoConv model with relationship models that use add_self_loops the message passing will be incorrect. For example:

HeteroConv({('A', 'to', 'B'): GATConv((-1, -1), hidden_channels, heads=4, dropout=0.2)})

This is caused by the fact that the self loops would then refer links between source and destination nodes of different types.

The suggestion here is to add a check inside HeteroConv that would warn raise an exception to avoid users unknowingly doing this.

[rusty1s]

Can we do the same for to_hetero()?