Add optimization callbacks that fire on a marker function

[vchuravy]

Fix #620
alternative for #599

[vchuravy]

This doesn't suffice.

Essentially Enzyme, needs a sequence of calls that work on the CPU and GPU.
With the current autodiff_deferred implementation we have that but reusing session pointers as keys into the deferred work-queue, and those session pointers are trampolines for the CPU.

So we might still need #599 and then this PR ought to not be necessary.

ptr = gpucompiler.lookup(Enzyme, f, ad_args...)
ccall(ptr, f, ad_args...)

And then Enzyme could implement gpucompiler(::Enzyme, f, ad_args...) to perform the necessary lookup on the CPU,
and we could use the abstract interpreter to perform the same on the GPU