Progress on nested scenarios for Flux

[gdalle]

Core

• Improve all the my... functions with Functors

Tests

• Add more complicated nested scenarios, test them on Zygote and Tracker

Extensions

• ForwardDiff: use Functors on dual-specific functions to be more generic
• Tracker: use withgradient by default (more modern), with a fallback for number inputs

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 78.08219% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 93.84%. Comparing base (36d9c47) to head (11e065f).

Files	Patch %	Lines
src/utils.jl	55.00%	9 Missing ⚠️
...xt/DifferentiationInterfaceForwardDiffExt/utils.jl	56.25%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
- Coverage   94.51%   93.84%   -0.68%     
==========================================
  Files          58       58              
  Lines        1477     1526      +49     
==========================================
+ Hits         1396     1432      +36     
- Misses         81       94      +13     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.