`LKJCholesky` model fails through `LogDensityProblems` when using `SimpleVarInfo`

[sunxd3]

using DynamicPPL, Distributions, LogDensityProblems

@model demo_lkj() = x ~ LKJCholesky(2, 1.0)
model = demo_lkj()

logdensity = DynamicPPL.LogDensityFunction(model, DynamicPPL.getlogjoint, DynamicPPL.VarInfo(model));
logdensity_simple = DynamicPPL.LogDensityFunction(model, DynamicPPL.getlogjoint, DynamicPPL.SimpleVarInfo(model));
θ = rand(4);

# this is fine
LogDensityProblems.logdensity(logdensity, θ)

# errors
LogDensityProblems.logdensity(logdensity_simple, θ)


ERROR: MethodError: no method matching length(::LinearAlgebra.Cholesky{Float64, Matrix{Float64}})

Closest candidates are:
  length(::PrettyTables.RowTable)
   @ PrettyTables ~/.julia/packages/PrettyTables/f6dXb/src/tables.jl:109
  length(::Distributions.VonMisesFisherSampler)
   @ Distributions ~/.julia/packages/Distributions/nmUhn/src/samplers/vonmisesfisher.jl:20
  length(::LibGit2.GitBlob)
   @ LibGit2 ~/.julia/juliaup/julia-1.10.5+0.aarch64.apple.darwin14/share/julia/stdlib/v1.10/LibGit2/src/blob.jl:3
  ...

Stacktrace:
 [1] map
   @ ./tuple.jl:291 [inlined]
 [2] unflatten(original::Tuple{LinearAlgebra.Cholesky{Float64, Matrix{Float64}}}, x::Vector{Float64})
   @ DynamicPPL ~/DynamicPPL.jl/src/utils.jl:628
 [3] unflatten(original::@NamedTuple{x::LinearAlgebra.Cholesky{Float64, Matrix{Float64}}}, x::Vector{Float64})
   @ DynamicPPL ~/DynamicPPL.jl/src/utils.jl:639
 [4] unflatten(svi::SimpleVarInfo{@NamedTuple{…}, Float64, DynamicPPL.NoTransformation}, x::Vector{Float64})
   @ DynamicPPL ~/DynamicPPL.jl/src/simple_varinfo.jl:264
 [5] unflatten(varinfo::SimpleVarInfo{…}, context::DefaultContext, θ::Vector{…})
   @ DynamicPPL ~/DynamicPPL.jl/src/abstract_varinfo.jl:737
 [6] logdensity(f::DynamicPPL.LogDensityFunction{SimpleVarInfo{…}, Model{…}, Nothing}, θ::Vector{Float64})
   @ DynamicPPL ~/DynamicPPL.jl/src/logdensityfunction.jl:139
 [7] top-level scope
   @ REPL[14]:1
Some type information was truncated. Use `show(err)` to see complete types.

versions
Julia 1.10.5
Bijectors: 1.13.17
DynamicPPL: 0.29.0

(Still an issue on 0.37.0)

[penelopeysm]

No longer relevant in 0.39 as you can't construct a LDF with a SVI (even if you could, it wouldn't use this code path) 😎