Fix Gibbs/submodel/template handling bug

[penelopeysm]

Closes #2798

[github-actions]

Turing.jl documentation for PR #2799 is available at:
https://TuringLang.github.io/Turing.jl/previews/PR2799/

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.45%. Comparing base (8410b09) to head (211dc4c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mcmc/gibbs.jl	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2799      +/-   ##
==========================================
- Coverage   86.45%   86.45%   -0.01%     
==========================================
  Files          22       22              
  Lines        1447     1454       +7     
==========================================
+ Hits         1251     1257       +6     
- Misses        196      197       +1     

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