[AutoDiff] Fix differentiable_function-related specialization crashes.

[dan-zheng]

Fix crashes related to generic specialization of partial_apply operands to
differentiable_function instructions.

differentiable_function requires derivative function operand types to match
expected derivative function types computed from the original function operand's
type, so operands cannot be specialized individually without specializing the
others.

Resolves TF-891 and TF-1126.

---

Fixed verification error from TF-1126:

Looking for a function: $ss4SIMDPss14DifferentiableRzSB6Scalars11SIMDStoragePRpzsAA13TangentVectorsACPRpzSBAhI_AdFRPzrlE12_vjpSubtract3lhs3rhsx5value_AJ_AJtAJc8pullbacktx_xtFZs5SIMD8VySfG_Tg5
Expected type: @convention(method) (@in_guaranteed SIMD8<Float>, @in_guaranteed SIMD8<Float>, @thick SIMD8<Float>.Type) -> (@out SIMD8<Float>, @owned @callee_guaranteed (@in_guaranteed SIMD8<Float>) -> (@out SIMD8<Float>, @out SIMD8<Float>))
Found    type: @convention(method) (SIMD8<Float>, SIMD8<Float>, @thick SIMD8<Float>.Type) -> (@out SIMD8<Float>, @owned @callee_guaranteed (@in_guaranteed SIMD8<Float>) -> (@out SIMD8<Float>, @out SIMD8<Float>))
Assertion failed: (ReInfo.getSpecializedType() == SpecializedF->getLoweredFunctionType() && "Previously specialized function does not match expected type."), function lookupSpecialization, file /Users/swiftninjas/s4tf/swift/lib/SILOptimizer/Utils/Generics.cpp, line 1827.

[dan-zheng]

@swift-ci Please test tensorflow

[marcrasi]

Ooo nice.

Seems like we'll want to allow specialization of these things eventually because specialization can make things much faster?

[rxwei]

I think eventually we'll want specialization to apply to the @differentiable function, not operands to the differentiable_function instruction.

[dan-zheng]

Seems like we'll want to allow specialization of these things eventually because specialization can make things much faster?

Yes, allowing specialization if possible would be ideal!

A nice approach might be to relax differentiable_function/differentiable_function_extract verification while preserving global derivative function type consistency, à la #27923.

This avoids requiring that all function-type-rewriting transformations (e.g. generic specialization, loadable-by-address) must update original/derivative function types consistently.