Add common interface for pullback functions

[adrhill]

Reverse-mode backends like Zygote and Enzyme (?) are able to return pullback functions / closures instead of just the return value of the pullback, which is currently returned by value_and_pullback!.

DI could offer a common interface value_and_pullback_fn (or a mutating value_and_pullback!_fn) for users in need of such closures.

[gdalle]

At first I thought we would be able to pull this off with the prepare mechanism, but now I see that prepare is specific for different inputs, whereas we reuse a pullback for the same input. So you have my go ahead ^^

[gdalle]

Here's what I spotted:

Forward mode

• Diffractor: the pushforward seems to be the fundamental object, used in the AbstractDifferentiation primitive

Reverse mode

• ChainRulesCore: pullback closure generated by frule_via_ad
• Enzyme.jl: tape generated by autodiff_thunk (see Enzyme.jl#1335)
• Tracker: pullback closure generated by forward, see README or source code e.g. here
• Zygote: pullback closure generated by pullback