I saw that you have a proposal for get_logdensity , get_gradient and get_payload functions in there, so that would go in the direction that @ChrisRackauckas proposed.
But I think it would be important to have this in a very lightweight central package that does nothing else (LogDensityProblems, for example, while I like it, does have it’s own opinions on how to run AD frameworks and has a load time of 7.5s - not a dependency someone will want to take on just to define a likelihood).
Would there be interest in creating such a standard package that just defines a pure API of functions to access target-function result values?