Use new stan::math::gradient when available about bridgestan

[roualdes]

I think *val can be used directly in stan::math::gradient within log_density and similar functions, instead of creating double lp and then exchanging the value as in

bridgestan/src/model_rng.cpp

Line 197 in 2f730e1

stan::math::gradient(logp, params_unc, lp, grad_vec);

.

[WardBrian]

I think you’re right, that was probably missed during the change to return codes

[bob-carpenter]

OK, I'll submit a patch.

[WardBrian]

I’ve actually been running locally against CmdStan develop, which means today I needed to update some things for the new location of the JSON parser. I also took a crack at this while I was at it: https://github.com/roualdes/bridgestan/tree/stan-2-31

[roualdes]

Nice. That this change is so seamless for BridgeStan is excellent.

Assuming this is just a prototype, I'll save my comments until a future PR.

Do you know off the top of your head when (roughly) 2.31 is likely to be released?

[WardBrian]

The releases are ~4 months apart, so the feature freeze should start toward the end of October and the release itself should be early November.