tree 40eee074730690ac2fd3e27cb9de3da72eb8ffee
parent 3d1b494dce4c3eeb4ee493fca7a481c717d30000
author Sameer Agarwal <sameeragarwal@google.com> 1773784099 -0700
committer Sameer Agarwal <sameeragarwal@google.com> 1774206375 -0700

[modernize] Modernize internal autodiff and numeric_diff logic

Refactor the core automatic and numeric differentiation internal helpers
to utilize C++17 features for improved readability and maintainability:

- Added IntegerSequenceTraits to provide a uniform compile-time interface
  for accessing head and tail of std::integer_sequence.
- Eliminated recursive template meta-programming in autodiff.h, replacing
  it with fold expressions and std::index_sequence.
- Simplified core helper functions like Make1stOrderPerturbation into
  unrolled loops.
- Refactored EvaluateJacobianForParameterBlocks from a recursive struct
  to a recursive function template using if constexpr.
- Updated NumericDiffCostFunction and NumericDiffFirstOrderFunction
  to use the modernized internal helper API.

Change-Id: I934034f7434fc05a8855565b2b534d325f920584