ForwardDiff over anything - prepare inner gradient for HVP

[gdalle]

DI source

• Move, rename and (partially) document functors Gradient and PushforwardFixedSeed

DI extensions

• ForwardDiff: add secondorder.jl file allowing preparation of the inner gradient within an HVP. The secret is to figure out the dual type that will be passed during the pushforward, and to give this type to prepare_gradient. This requires fixing the ForwardDiff tag so that it doesn't depend on the gradient closure (which results in a circular dependency, see Preparation for second order #86).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.92308% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.62%. Comparing base (1c35de5) to head (f3264a5).

Files	Patch %	Lines
...rentiationInterface/src/first_order/pushforward.jl	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #385      +/-   ##
==========================================
+ Coverage   96.59%   96.62%   +0.03%     
==========================================
  Files         101      102       +1     
  Lines        4929     4974      +45     
==========================================
+ Hits         4761     4806      +45     
  Misses        168      168              

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