[AutoDiff upstream] add @noDerivative to AnyFunctionType params

[marcrasi]

Adds the @noDerivative attribute to AnyFunctionType parameters.

This attribute is used to determine which parameters a differentiable function type is differentiable with respect to, e.g. @differentiable (Float, @noDerivative Float) -> Float is only differentiable with respect to its first parameter.

Also adds serialization for the attribute.

This resolves TF-870.

[lattner]

The name @nondiff is a bit weird. Was @nondifferentiable considered?

[lattner]

Well, that isn't right either, because it is possible differentiable but you don't want it in this case - better names still desired :)

[lattner]

this LGTM, but the name @nondiff isn't great. For the time being, I'd suggest going with @nondifferentiable to align with the name inside of the compiler unless there is a better suggestion.

[marcrasi]

I discussed the naming with @dan-zheng, and we think that @noDerivative would be a pretty good name, because the meaning is very similar to the @noDerivative attribute on struct fields. For example, if you wanted to explode a struct in a function type, keeping @noDerivative on the corresponding fields is exactly what you need to do:

struct Foo {
  let a: Float
  @noDerivative let b: Float
}

let f: @differentiable (Foo) -> Float
let g: @differentiable (Float, @noDerivative Float) -> Float

It's already named @nondiff in the tensorflow branch, and various client code uses it, so I'm going to have to do a deprecate-then-remove thingy, but this shouldn't be too hard.

cc @rxwei for opinions.

I'll wait a bit to see if anyone disagrees before I make the code changes.

[rxwei]

I think we should use @noDerivative.

[marcrasi]

I have renamed this to @noDerivative. I will run tests but wait until Monday to merge, to avoid causing any postsubmit CI failures over the weekend.

[marcrasi]

@swift-ci please test

[marcrasi]

@swift-ci please test

[lattner]

+1 for @noDerivative !

[marcrasi]

I have resolved merge conflicts, and I plan to do a test and merge soon.

[marcrasi]

@swift-ci please test and merge

[marcrasi]

@swift-ci please test and merge