C++ interface TODO list

I'm writing this issue to capture some of the ideas mentioned in a private discussion with @wsmoses about the future C++ interface. I'm hoping that this will increase visibility and allow other contributors to weigh in with design suggestions and considerations, as well as track progress on these topics.

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1. ~(fix WIP) Address an issue where C++ functions that return composite types by-value are mishandled after `sret` transformations (and produce error messages that report the functions return `void`). This has been observed in some larger projects, but still needs a minimal reproducer.~
edit: fixed by https://github.com/EnzymeAD/Enzyme/pull/2051

2. Add a `make_zero` function for basic types (and allow users to specialize it for custom types), in order to better support differentiation over custom containers.

3. Add higher-level tools for Jacobian, Hessian, JVP, VJP, HVP (actual interface still TBD, maybe look to Julia implementation for inspiration here?)

4. Support for batch-duplicated arguments (what container / layout should be used to pass the batch?)

5. Custom Derivative rule interface (likely also requires some effort in front end)