Remove `_Differentiable.zeroTangentVectorInitializer` to address the feedback on the [proposal thread](https://forums.swift.org/t/differentiable-programming-for-gradient-based-machine-learning/42147). The corresponding change has already been made in the [proposal](https://github.com/rxwei/swift-evolution/blob/autodiff/proposals/0000-differentiable-programming.md). Removed components: - `zeroTangentVectorInitializer` and `zeroTangentVector` in `Differentiable`, `Array`, `Optional`, `Float`, `Double`, `Float80`, and SIMD types. - `zeroTangentVectorInitializer` synthesis logic in `Differentiable` derived conformances.
Optional
Differentiable
Make `Optional` conditionally conform to `Differentiable` when the `Wrapped` type does. `Optional.TangentVector` is a wrapper around `Wrapped.TangentVector?`. Also, fix `Array.TangentVector.zeroTangentVectorInitializer`. Resolves TF-1301.
