fix: replace use of undocumented AutoForwardDiff constructor

[gdalle]

Fixes #628

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.52%. Comparing base (34e0cfc) to head (554ece9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #629   +/-   ##
=======================================
  Coverage   97.52%   97.52%           
=======================================
  Files         111      111           
  Lines        5529     5529           
=======================================
  Hits         5392     5392           
  Misses        137      137           

Flag	Coverage Δ
DI	98.65% <100.00%> (ø)
DIT	95.35% <ø> (ø)

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