We've recently changed the type checker to improve the ergonomics of the @differentiable attribute.

• Update TypeCheckAttr to infer what not to differentiate wrt for non-differentiable types. swiftlang/swift#22915: Explicit differentiation parameters are no longer required in a @differentiable attribute when the function has some arguments that do not conform to Differentiable. Those non-differentiable parameters will be skipped and the rest will be differentiated with respect to.
• [AutoDiff] TF-{295,285}: Make '@differentiable' default parameters include 'self' on instance methods. swiftlang/swift#22877: On an instance method, when a wrt: is not specified, self is being implicitly included as a differentiation parameter.
• [AutoDiff] TF-{295,285}: Make '@differentiable' default parameters include 'self' on instance methods. swiftlang/swift#22877: When a @differentiable requirement is not met, the @differentiable attribute fix-it will appear exactly as written in the original declaration instead of the most complex, canonical form. For instance, @differentiable instead of @differentiable(wrt: (x)).

This greatly simplifies libraries and applications that use automatic differentiation. The protocol requirement Layer.applied(to:in:) becomes as simple as this:

@differentiable
func applied(to input: Input, in context: Context) -> Output

This PR updates deep learning APIs to use the simplest form of @differentiable possible. Hooray!