Adds GATv2 layer

[abieler]

• Adds GATv2 layer from https://arxiv.org/abs/2105.14491
• fixes typo in GATConv

IDK if its worth introducing a new abstact type for the two GAT layers as some of the functions are identical.

Could replicate the results from Figure 1 in the paper regarding the attention scores:

GATConv Layer

GATv2 Layer

note the difference is that here we have self-loops required, which results in the 1st alpha score not being 0 for all queries.

[yuehhua]

Would you mind adding some tests?

[yuehhua]

@abieler Thank you for contribution. I will merge this first then fix the tests.