[AutoDiff] Rename "associated function" to "derivative function".

[rxwei]

assocFn -> derivativeFn
AssocFn -> DerivativeFn
assocFunc -> derivativeFunc
AssocFunc -> DerivativeFunc
associatedFunction -> derivativeFunction
AssociatedFunction -> DerivativeFunction
autodiff associated function -> derivative function
autodiff-associated function -> derivative function
AD associated function -> derivative function
associated differentiation function -> derivative function

This is a follow-up to #27597.

Resolves TF-882.

[dan-zheng]

👍 the renaming!

Verified no remaining results:

$ grep -nri "an derivative" include lib docs test
$ grep -nri "associated function" include lib docs test
$ grep -nri "assoc function" include lib docs test
$ grep -nri "assocfn" include lib docs test
$ grep -nri "assoc derivative" include lib docs test

associated derivative remains but I think it's okay:

$ grep -nri "associated derivative" include lib docs test
include/swift/AST/Attr.h:1529:/// custom associated derivative functions: 'jvp' and 'vjp'.
test/AutoDiff/forward_mode_runtime.swift:445:// Test case where associated derivative function's requirements are met.
test/AutoDiff/generics.swift:31:// Test case where associated derivative function's requirements are met.

[rxwei]

@swift-ci please test tensorflow

[rxwei]

Thanks for verifying!