Implement support for `ChainRules.frule_via_ad` / `rrule_via_ad`

[yebai]

It would be helpful to allow others to use Enzyme as a tool for automatically generating ChainRules rules. See:

Writing rules that call back into AD

Related issue: chalk-lab/Mooncake.jl#848

[wsmoses]

I don't quite understand the ask, but contributions welcome!

[yebai]

The idea is explained very clearly in the ChainRules documentation — it would enable interoperability between different automatic differentiation (autodiff) backends. For example, autodiff backends compatible with ChainRules could use Enzyme to differentiate through specific functions by importing the ChainRules generated by Enzyme.

It might be difficult for someone who isn’t a core Enzyme developer to implement this, although it could be feasible with sufficient guidance from a core Enzyme developer.

[wsmoses]

yeah I get the goal of that from the users perspective, but not what APIs need to be implemented to enable.

In any case, since it sounds like you have a good understanding (and this seemingly ought only call into Enzyme from a user perspective of calling autodiff/gradient/autodiff_thunk/etc), feel free to make a draft PR and we can discuss there how to complete