Support for DimeNet++ model

[arunppsg]

🚀 The feature, motivation and pitch

The DimeNet model is a model which can be used for molecular property prediction. DimeNet++ model is an upgrade to the DimeNet model. DimeNet++ model is faster than DimeNet and more accurate (ref). It will be nice to have DimeNet++ model in PyG.

There are two possible ways to add DimeNet++:

1. The key difference between DimeNet and DimeNet++ is the InteractionBlock (DimeNet++ uses an updated version of InteractionBlock - called as InteractionPPBlock). Also, the authors proposed a minor upgrade to OutputBlock in DimeNet++ (OutputPPBlock). The forward method are same in both. Thus, we can subclass DimeNet++ to be a child class of DimeNet, and add an option in DimeNet __init__ method to use DimeNet++ architecture rather than DimeNet architecture (something like plusplus = True, then initialize plusplus model, thereby using OutputPPBlock and InteractionPPBlock, we also need to add a couple of additional parameters required by DimeNet++).
2. Create an independent implementation of DimeNet++. There is a pytorch implementation here which I adapted for PyG. The adapted version can be found here.

Thanks for consideration! Let me know if I can provide any other additional details.

[rusty1s]

Yes, I always wanted to support DimeNet++ as well. Sub-classing sounds like the way to go for me. We also have an open PR regarding this #2421 but I hadn't have a deeper look yet.

Let me know if you think parts of this are re-usable. Otherwise, we can start from scratch :)

[arunppsg]

Oops, I didn't see the earlier issue. I too prefer subclassing. Here is my brief approach:

• Let's upgrade the current OutputBlock to use the architecture proposed in DimeNet++. By this, DimeNet will also use OutputBlock presented in DimeNet++ block but I think this is fine since it is also beneficial DimeNet.
• We will implement an new InteractionPPBlock
• Add an user argument for plusplus and we can initialize variables accordingly

Also, we can add example and tests. I will put up a PR later today or by this week.