[AutoDiff] Derive Differentiable.zeroTangentVectorInitializer.

[dan-zheng]

Cherry-pick of #31823 from master branch.

---

Differentiable conformance derivation now supports
Differentiable.zeroTangentVectorInitializer.

zeroTangentVectorInitializer is a closure that produces a zero tangent vector,
capturing minimal necessary information from self.

It is an instance property, unlike the static property AdditiveArithmetic.zero,
and should be used by the differentiation transform for correctness.

There are two potential derivation cases:

1. Memberwise derivation: done when TangentVector can be initialized memberwise.
2. { TangentVector.zero } derivation: done as a fallback.

Remove Differentiable.zeroTangentVectorInitializer dummy default implementation.

Update stdlib Differentiable conformances and tests.
Clean up DerivedConformanceDifferentiable.cpp cruft.

Resolves TF-1007.
Progress towards TF-1008: differentiation correctness for projection operations.

---

Examples:

import _Differentiation

struct Struct<T: Differentiable, U: Differentiable>: Differentiable {
  var x: T
  var y: U

  // Compiler synthesizes:
  // var zeroTangentVectorInitializer: () -> TangentVector {
  //   { [xZeroTanInit = x.zeroTangentVectorInitializer,
  //      yZeroTanInit = y.zeroTangentVectorInitializer] in
  //     return TangentVector(x: xZeroTanInit(), y: yZeroTanInit())
  //   }
  // }
}

let s = Struct(x: [1, 2, 3], y: [[4, 5, 6], [], [2]])
// `Differentiable.zeroTangentVector` default implementation calls
// `zeroTangentVectorInitializer`.
print(s.zeroTangentVector)

// Before (via dummy `zeroTangentVectorInitializer` default implementation):
// TangentVector(x: [], y: [])

// After (via derived conformances):
// TangentVector(x: [0.0, 0.0, 0.0], y: [[0.0, 0.0, 0.0], [], [0.0]])

struct CustomTangentVector<T: Differentiable, U: Differentiable>: Differentiable {
  var x: T
  var y: U

  typealias TangentVector = T.TangentVector
  mutating func move(along direction: TangentVector) {}

  // Memberwise synthesis is not possible for custom `TangentVector` type.
  // Fallback synthesis is done instead:
  // var zeroTangentVectorInitializer: () -> TangentVector {
  //   { TangentVector.zero }
  // }
}

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

@swift-ci Please clean test TensorFlow