Add huber loss support

[jrabary]

This PR adds huber loss support. Used for object detection for instance.

[eaplatanios]

Thanks @jrabary for addressing the comments promptly!:)

[dan-zheng]

LGTM after comments are addressed!

If you enable "Allow edits from maintainers" on the PR, I'm happy to push edits directly.

[dan-zheng]

CI failed:

/swift-apis/Sources/TensorFlow/StdlibExtensions.swift:217:1: error: type 'Element' does not conform to protocol 'Differentiable'
extension Array.DifferentiableView: VectorProtocol where Element: VectorProtocol {
^
/swift-apis/Sources/TensorFlow/StdlibExtensions.swift:217:1: error: 'VectorProtocol' requires that 'Element' conform to 'Differentiable'
extension Array.DifferentiableView: VectorProtocol where Element: VectorProtocol {
^
/swift-apis/Sources/TensorFlow/StdlibExtensions.swift:217:1: note: requirement specified as 'Element' : 'Differentiable'
extension Array.DifferentiableView: VectorProtocol where Element: VectorProtocol {
^
/swift-apis/Sources/TensorFlow/StdlibExtensions.swift:217:1: note: requirement from conditional conformance of 'Array<Element>.DifferentiableView' to 'AdditiveArithmetic'
extension Array.DifferentiableView: VectorProtocol where Element: VectorProtocol {
...

Could you please trying merging from master? I believe that should fix the errors.

[dan-zheng]

Thank you!