Remove default_varinfo function

[penelopeysm]

default_varinfo isn't used anywhere else in TuringLang and this PR helps to remove one level of indirection.

Also updated the comment to help provide some context.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.20%. Comparing base (54691bf) to head (23358b4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #695      +/-   ##
==========================================
- Coverage   79.22%   79.20%   -0.03%     
==========================================
  Files          30       30              
  Lines        4212     4207       -5     
==========================================
- Hits         3337     3332       -5     
  Misses        875      875              

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[coveralls]

Pull Request Test Coverage Report for Build 11394594173

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 73 unchanged lines in 17 files lost coverage.
• Overall coverage decreased (-0.02%) to 79.599%

---

Files with Coverage Reduction	New Missed Lines	%
src/simple_varinfo.jl	1	89.18%
src/sampler.jl	1	92.0%
ext/DynamicPPLEnzymeCoreExt.jl	1	0.0%
src/contexts.jl	3	75.76%
src/context_implementations.jl	3	63.67%
src/test_utils.jl	3	85.31%
src/model_utils.jl	3	19.64%
src/varnamedvector.jl	3	89.42%
src/distribution_wrappers.jl	4	44.12%
ext/DynamicPPLForwardDiffExt.jl	4	66.67%

Totals
Change from base Build 11381380435:	-0.02%
Covered Lines:	3332
Relevant Lines:	4186

---

💛 - Coveralls

[torfjelde]

So this was deliberately introduced at some point exactly to allow using other AbstractVarInfo implementations for a given sampler by overloading this method. It's true that it's not used anywhere, but I have personally used this method in the past to use SimpleVarInfo for a specific model because it lead to performance improvements. Sooo I'm somewhat reluctant to remove it 🤷 But yeah, up for discussion.

[penelopeysm]

@torfjelde, I'm sorry, you stumbled across a PRs which is definitely not intended for review yet 😅 I've been trying to see how much of the codebase I can strip out without CI failing (hence a few force pushes). I will definitely consult for opinions before asking for proper reviews!

[torfjelde]

Ah gotcha, gotcha:) Seems very sensible!