Add `getmodel` and `setmodel` from/to `LogDensityFunction`

[sunxd3]

ref: https://github.com/TuringLang/Turing.jl/pull/2231#issuecomment-2192601314

[sunxd3]

@yebai @torfjelde I don't think we need this PR now. The motivation of this PR was that we want customized setmodel for different AD backend. We more or less established at https://github.com/TuringLang/Turing.jl/pull/2231#issuecomment-2192601314 that we will postpone this until we need it.

[yebai]

@sunxd3 let's still transfer these methods from Turing into DynamicPPL so we can maintain them from DynamicPPL.

[coveralls]

Pull Request Test Coverage Report for Build 9992511738

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 24 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.03%) to 81.41%

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Files with Coverage Reduction	New Missed Lines	%
src/model.jl	1	87.0%
src/simple_varinfo.jl	5	82.18%
src/varinfo.jl	6	84.85%
src/threadsafe.jl	12	46.73%
<!--	Total:	24		-->

Totals
Change from base Build 9945722994:	0.03%
Covered Lines:	2807
Relevant Lines:	3448

---

💛 - Coveralls

[sunxd3]

@torfjelde another look? 🙏

[torfjelde]

LGTM:)