Use DI preparation result when initializing Jacobian

[gdalle]

This should fix #469 for the example you gave.

[avik-pal]

Ah my bad, completely missed it. Thanks!

[gdalle]

It would be fair to expect DI.jacobian(b, AutoForwardDiff(; chunksize=2), x) to be type-stable. The problem is that this falls back directly onto ForwardDiff.jacobian(f, x), which is itself type-unstable since I don't give it the static chunk size it needs to create the JacobianConfig. I could give it the chunk size, but I would have to create a JacobianConfig myself for that, which is unnecessary work e.g. for StaticArrays.

[ChrisRackauckas]

We allowed for specifying chunk size as a Val to handle that. We probably need to ensure that's allowed in ADTypes implementations.

[gdalle]

This should solve it: JuliaDiff/DifferentiationInterface.jl#541