Flux Compatibility

You may have noticed that I recently overhauled Flux's AD to emphasise a more functional API. My not-so-secret goal here is that Flux's current AD will be forwards-compatible with the better infrastructure that we are building; it should be easy to swap Capstan into an existing model without any code changes.

For the most part, this isn't any of Capstan's problem. But we do need Capstan to be able to strip away Flux's tracking before applying its own. Capstan already does checks whenever a new value is introduced to the program (for the wrt! API), so this should be as simple as applying a function like unwrap just after that –

unwrap(x) = x
# In Flux
unwrap(x::TrackedArray) = x.data

We can use this issue to discuss any other compatibility issues that might come up; it might be useful to start having some Flux examples, and I'm happy to help get that set up.

Start by locating Capstan's wrt! handling and the point where new values are introduced. Read the proposed unwrap behavior and consider the Flux compatibility examples mentioned in the issue; done means Flux-tracked values can be stripped before Capstan applies its own automatic differentiation.