Optimize Enzyme gradient

[gdalle]

DI extensions

Enzyme:

• Improve gradient! by using preparation
• Implement versions of gradient without preparation

Blocked by EnzymeAD/Enzyme.jl#1929 (I had just forgotten the activity annotation)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.70%. Comparing base (7a09754) to head (0cd1877).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #515      +/-   ##
==========================================
- Coverage   98.71%   98.70%   -0.02%     
==========================================
  Files         105      105              
  Lines        4523     4545      +22     
==========================================
+ Hits         4465     4486      +21     
- Misses         58       59       +1     

Flag	Coverage Δ
DI	98.87% <100.00%> (-0.03%)	⬇️
DIT	98.31% <ø> (ø)

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