.~ is broken for MvNormal

[xukai92]

using Turing

@model mwe(::Type{T}=Float64) where {T<:Real} = begin
    m = Matrix{T}(undef, 2, 3)
    m .~ MvNormal(zeros(2), 1)
end

sample(mwe(), HMC(0.2, 4), 1_000)

gives

MethodError: no method matching dim(::MvNormal{Float64,PDMats.ScalMat{Float64},Array{Float64,1}})
Closest candidates are:
  dim(!Matched::PDMats.PDMat) at /afs/inf.ed.ac.uk/user/s16/s1672897/.julia/packages/PDMats/nKT0N/src/pdmat.jl:30
  dim(!Matched::PDMats.PDSparseMat) at /afs/inf.ed.ac.uk/user/s16/s1672897/.julia/packages/PDMats/nKT0N/src/pdsparsemat.jl:30
  dim(!Matched::PDMats.PDiagMat) at /afs/inf.ed.ac.uk/user/s16/s1672897/.julia/packages/PDMats/nKT0N/src/pdiagmat.jl:26
  ...

[mohamed82008]

Should be fixed in TuringLang/DistributionsAD.jl#19.