Allow specification of parameters to test for in kwargs

[torfjelde]

Some AD backends, e.g. ReverseDiff, have a compiled mode which relies on the particular input value provided in the ADgradient call. Since we're currently just checking gradient evaluations for the same input values as the one we use for compilation of the tape, we don't catch issues related to this.

This PR just makes the input values we run benchmarks on part of the kwargs, with the default values differing from the values present in the varinfo we use for compilation.

Related discussion: TuringLang/Turing.jl#2091 (comment)

Example from that discussion is now correctly identified to have issues:

julia> using Turing, TuringBenchmarking, Random

julia> @model demo() = L ~ LKJCholesky(3, 1.0)

demo (generic function with 2 methods)

julia> model = demo();

julia> make_turing_suite(model; check=true, adbackends=[:reversediff, :reversediff_compiled]);
┌ Warning: There is disagreement in the gradients!
└ @ TuringBenchmarking /drive-2/Projects/public/TuringBenchmarking.jl/src/TuringBenchmarking.jl:264
┌──────────────────────────────────────┬──────────┐
│                               Linked │ Gradient │
│                              backend │ distance │
├──────────────────────────────────────┼──────────┤
│ ReverseDiff[compiled] vs ReverseDiff │     0.31 │
└──────────────────────────────────────┴──────────┘

[yebai]

Maybe consider replacing θ_linked with is_linked, and internally compute θ_linked from θ (or compute θ from θ_linked, when is_linked is true) when needed.

This prevents the user from accidentally passing inconsistent θ and θ_linked.

[torfjelde]

The reason why I didn't do this is because it's not really possible to do this in a completely consistent way (we don't know how the user has constructed the varinfo).

We could implement _default_params_linked(model, varinfo, theta) which would set the values in varinfo to theta and then link, but this would only work properly if theta is consistent with varinfo. So, I think this is just simpler (even though it pushes some of the effort onto the user).

[yebai]

Looks good except for the minor comment above.