Fix ParameterlessLayer conformances.

[dan-zheng]

Associated type inference behavior was changed in swiftlang/swift#32578:
derived conformances are now attempted before associated type inference.

This broke ParameterlessLayer, which relied on a
TangentVector == EmptyTangentVector same-type constraint to set a
default TangentVector type witness for conforming types.

Add explicit TangentVector type witnesses to
ParameterlessLayer-conforming types to fix this regression.

This workaround is forward- and backward-compatible, but makes code
more verbose.

---

This regression after swiftlang/swift#32578 is unfortunate, especially because
there doesn't seem to exist a workaround that doesn't produce a warning.

I'll write a minimal regression reproducer and ask on the PR if there's some principled
way to support the old behavior.

---

Fixes build errors from the 2020-06-30 master -> tensorflow merge:

tensorflow-swift-apis/Sources/TensorFlow/Layers/Core.swift:21:15: error: 'ParameterlessLayer' requires the types 'Flatten<Scalar>.TangentVector' and 'EmptyTangentVector' be equivalent
public struct Flatten<Scalar: TensorFlowFloatingPoint>: ParameterlessLayer {
              ^
tensorflow-swift-apis/Sources/TensorFlow/Layers/Core.swift:21:15: note: requirement specified as 'Self.TangentVector' == 'EmptyTangentVector' [with Self = Flatten<Scalar>]
public struct Flatten<Scalar: TensorFlowFloatingPoint>: ParameterlessLayer {
              ^
tensorflow-swift-apis/Sources/TensorFlow/Layers/Core.swift:21:15: error: type 'Flatten<Scalar>.TangentVector' does not conform to protocol 'VectorProtocol'
public struct Flatten<Scalar: TensorFlowFloatingPoint>: ParameterlessLayer {
              ^
tensorflow-swift-apis/Sources/TensorFlow/Layers/Core.swift:39:15: error: 'ParameterlessLayer' requires the types 'Reshape<Scalar>.TangentVector' and 'EmptyTangentVector' be equivalent
public struct Reshape<Scalar: TensorFlowFloatingPoint>: ParameterlessLayer {
              ^
tensorflow-swift-apis/Sources/TensorFlow/Layers/Core.swift:39:15: note: requirement specified as 'Self.TangentVector' == 'EmptyTangentVector' [with Self = Reshape<Scalar>]
public struct Reshape<Scalar: TensorFlowFloatingPoint>: ParameterlessLayer {
              ^
tensorflow-swift-apis/Sources/TensorFlow/Layers/Core.swift:72:15: error: 'ParameterlessLayer' requires the types 'Function<Input, Output>.TangentVector' and 'EmptyTangentVector' be equivalent
public struct Function<Input: Differentiable, Output: Differentiable>: ParameterlessLayer {
              ^
tensorflow-swift-apis/Sources/TensorFlow/Layers/Core.swift:72:15: note: requirement specified as 'Self.TangentVector' == 'EmptyTangentVector' [with Self = Function<Input, Output>]
public struct Function<Input: Differentiable, Output: Differentiable>: ParameterlessLayer {
              ^
tensorflow-swift-apis/Sources/TensorFlow/Layers/Core.swift:72:15: error: type 'Function<Input, Output>.TangentVector' does not conform to protocol 'VectorProtocol'
public struct Function<Input: Differentiable, Output: Differentiable>: ParameterlessLayer {
              ^