use `HessianConfig` to calculate gradients?

[longemen3000]

Hello,

Recently i came across with a problem in which already had a hessian cache, (HessianConfig + DiffResults.HessianResult), but it seems there aren't any methods for calculating for using HessianConfig (and HessianResult) when calculating gradients (of the same function with the same number and type of arguments).

I did this dirty hack to allow calculating gradients with HessianConfig + HessianResult:

hresult,hconfig = cache    
h_jconfig = hconfig.jacobian_config
seeds = h_jconfig.seeds
duals = h_jconfig.duals[1]
gconfig = ForwardDiff.GradientConfig{Nothing,eltype(aux),length(seeds),typeof(duals)}(seeds,duals)
gresult = ForwardDiff.MutableDiffResult(result.value,(result.derivs[1],))

It works for me, but it could be generalized (by passing the adequate function argument, for example)

It would be much easier to just do ForwardDiff.gradient!(hresult,f,x,hconfig) instead.