Fix type inference of `eltype(vi, spl)` (Turing#2151)

[devmotion]

Fixes the error in https://github.com/TuringLang/Turing.jl/issues/2151 on Julia 1.10 for me:

using Turing, ReverseDiff, LinearAlgebra

# build data
t = 1:0.05:8
σ = 0.3
y = @. rand(sin(t) + Normal(0, σ))

@model function state_space(y, TT, ::Type{T}=Float64) where {T}
    # Priors 
    α ~ Normal(y[1], 0.001)
    τ ~ Exponential(1)
    η ~ filldist(Normal(0, 1), TT-1)
    σ ~ Exponential(1)

    # create latent variable
    x = Vector{T}(undef, TT)

    x[1] = α

    for t in 2:TT
        x[t] = x[t-1] + η[t-1] * τ
    end

    # measurement model
    y ~ MvNormal(x, σ^2 * I)

    return x
end

n_iter = 1500
model = state_space(y, length(t))
chns = sample(model, NUTS(n_iter * 2, 0.8; adtype = AutoReverseDiff(true)), n_iter);

cc @sjwild

[torfjelde]

This looks good to me, but there's something weird happening with the tests; looking into this.

@devmotion Could you bump patch version?

[github-actions[bot]]

Pull Request Test Coverage Report for Build 7583034714

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 79.326%

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Totals
Change from base Build 7506113239:	-0.5%
Covered Lines:	2329
Relevant Lines:	2936

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💛 - Coveralls

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (c6e3b6b) 79.64% compared to head (4ff8324) 84.28%. Report is 1 commits behind head on master.

Files	Patch %	Lines
src/abstract_varinfo.jl	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #568 +/- ## ========================================== + Coverage 79.64% 84.28% +4.63% ========================================== Files 26 26 Lines 2938 3175 +237 ========================================== + Hits 2340 2676 +336 + Misses 598 499 -99 ```

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[coveralls]

Pull Request Test Coverage Report for Build 7628589803

• 4 of 5 (80.0%) changed or added relevant lines in 2 files are covered.
• 25 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.02%) to 84.635%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/abstract_varinfo.jl	3	4	75.0%
<!--	Total:	4	5	80.0%	-->

Files with Coverage Reduction	New Missed Lines	%
src/model.jl	1	87.88%
src/simple_varinfo.jl	5	79.64%
src/varinfo.jl	6	85.6%
src/threadsafe.jl	13	44.23%
<!--	Total:	25		-->

Totals
Change from base Build 7615074409:	0.02%
Covered Lines:	2677
Relevant Lines:	3163

---

💛 - Coveralls

[devmotion]

I think TuringLang/Turing.jl/pull/2156 will fix the test issues.

[devmotion]

@torfjelde The PR is ready for another round of review 🙂