Add the DeepDDI model

Dear @hzcheney,

• Please read the paper first. It is here.
• After that read the contributing guidelines.
• If there is an existing open source version of the model please take a look.
• ChemicalX is built on top of PyTorch 1.10. and torchdrug.
• A similar model is which uses to generate drug representations. Take a look at the layer definition here.
• The library heavily builds on top on torchdrug and molecules in batches are PackedGraphs.
• There is already a model class under ./chemicalx/models/
• Context features, drug level features and labels are all FloatTensors.
• Look at the examples and tests under ./examples/ and ./tests/.
• Add auxiliary layers as you see fit - please document these, add tests and add these layers to the main readme.md if needed.
• Add typing to the initialisation and forward pass.
• Non data dependent hyperparameters should have default values.
• Please add tests under ./tests/ and make sure that your model/layer is tested with real data.
• Write an example under ./examples/. What is the AUC on the test set? Is it reasonable?

[cthoyt]

So you could just submit the PR to take in whatever drug features are available from the data loader (currently Morgan fingerprints) and in future work we could add different featurizations to the data loader.

[hzcheney]

@hzcheney Are you going to open a PR with your code?

@benedekrozemberczki Yeah! I have already opened a PR and please review it!