Add the MHCADDI model

[benedekrozemberczki]

Dear @sebastiandro,

• 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 hyper-ammeters 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?