SIL differentiability witnesses are a new top-level SIL construct mapping
an "original" SIL function and derivative configuration to derivative SIL
functions.
This patch adds `SILDifferentiabilityWitness` IRGen.
`SILDifferentiabilityWitness` has a fixed `{ i8*, i8* }` layout:
JVP and VJP derivative function pointers.
Resolves TF-1146.
`TangentSpace` represents the tangent space of a type.
- For `Differentiable`-conforming types:
- The tangent space is the `TangentVector` associated type.
- For tuple types:
- The tangent space is a tuple of the elements' tangent space types, for the
elements that have a tangent space.
- Other types have no tangent space.
`TypeBase::getAutoDiffTangentSpace` gets the tangent space of a type.
`TangentSpace` is used to:
- Compute the derivative function type of a given original function type.
- Compute the type of tangent/adjoint values during automatic differentiation.
Progress towards TF-828: upstream `@differentiable` attribute type-checking.
The `@derivative` attribute registers a function as a derivative of another
function-like declaration: a `func`, `init`, `subscript`, or `var` computed
property declaration.
The `@derivative` attribute also has an optional `wrt:` clause specifying the
parameters that are differentiated "with respect to", i.e. the differentiation
parameters. The differentiation parameters must conform to the `Differentiable`
protocol.
If the `wrt:` clause is unspecified, the differentiation parameters are inferred
to be all parameters that conform to `Differentiable`.
`@derivative` attribute type-checking verifies that the type of the derivative
function declaration is consistent with the type of the referenced original
declaration and the differentiation parameters.
The `@derivative` attribute is gated by the
`-enable-experimental-differentiable-programming` flag.
Resolves TF-829.
