Test Enzyme on demo models / varinfos

[penelopeysm]

This is highly speculative and will fail for many combinations of models / varinfos. The Enzyme tests also take literal hours to run so IMO this cannot be merged into main. I'm mainly opening this so that we can look at it.

In the long term I think a reasonable compromise might be to run Enzyme only on TypedVarInfo (or whatever DynamicPPL's default varinfo type is in the future).

Note: This PR also comments out all non-Enzyme AD tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 3.98%. Comparing base (7060896) to head (409fc3b).

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HEAD has 6 uploads less than BASE

Flag	BASE (7060896)	HEAD (409fc3b)
	13	7

Additional details and impacted files

@@               Coverage Diff                @@
##           py/no-ldp-ad    #813       +/-   ##
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- Coverage         85.87%   3.98%   -81.89%     
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  Files                35      35               
  Lines              4205    4164       -41     
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- Hits               3611     166     -3445     
- Misses              594    3998     +3404     

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