Add PNA

[ckhui]

Implements a single convolutional layer of the Principal Neighbourhood Aggregation Networks in paper
"Principal Neighbourhood Aggregation for Graph Nets" https://arxiv.org/abs/2004.05718

• add PNA model using pytorch_geometry
• add example
• add test

[cenyk1230]

@ckhui Good job. Have you ever reproduced the results of the PNA model on some graph classification datasets?

[ckhui]

@ckhui Good job. Have you ever reproduced the results of the PNA model on some graph classification datasets?

I only tested the code with PROTEINS without any parameter tuning.

As attached is the comparison with diffpool.