Make Zygote extension depend on ForwardDiff too

[gdalle]

DI extensions

• Add ForwardDiff to the list of triggers for the Zygote extension, since it is needed by the HVP

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.12%. Comparing base (042d75f) to head (ce5fd10).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #303      +/-   ##
==========================================
- Coverage   96.42%   93.12%   -3.31%     
==========================================
  Files          71       71              
  Lines        3973     3971       -2     
==========================================
- Hits         3831     3698     -133     
- Misses        142      273     +131     

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[adrhill]

Does this mean Zygote can only be used if ForwardDiff is loaded as well?

[gdalle]

Yes. In practice it doesn't change anything because ForwardDiff is a dependency of Zygote anyway, and dropping it would be a breaking Zygote change so we'd see it coming