Better HVP in reverse over forward

[gdalle]

DI source

• Improve HVP for reverse-over-forward by putting the tangent in a Constant context

DI tests

• Add FromPrimitive tests for heterogeneous SecondOrder backends

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.42%. Comparing base (472c027) to head (125ea9a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #494      +/-   ##
==========================================
+ Coverage   98.30%   98.42%   +0.11%     
==========================================
  Files         106      106              
  Lines        4309     4309              
==========================================
+ Hits         4236     4241       +5     
+ Misses         73       68       -5     

Flag	Coverage Δ
DI	98.48% <100.00%> (+0.17%)	⬆️
DIT	98.28% <ø> (ø)

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