`Array.TangentVector` conformance to `AdditiveArithmetic` was incorrect as the returned values weren't negated if the lhs was an empty vector (considered to be a zero tangentvector)
Implement correct zero tangent vector, i.e. empty array [], handling during the backward pass of Array.DifferentiableView.+ and move methods. The precondition is no longer triggered by .zero/empty arrays.
Fixes SR-14297
Rename `move(along:)` to `move(by:)` based on the proposal feedback. The main argument for the change is that tangent vectors specify both a direction and a magnitude, whereas `along:` does not indicate that `self` is being moved by the specified magnitude.
Compiler:
- Add `Forward` and `Reverse` to `DifferentiabilityKind`.
- Expand `DifferentiabilityMask` in `ExtInfo` to 3 bits so that it now holds all 4 cases of `DifferentiabilityKind`.
- Parse `@differentiable(reverse)` and `@differentiable(_forward)` declaration attributes and type attributes.
- Emit a warning for `@differentiable` without `reverse`.
- Emit an error for `@differentiable(_forward)`.
- Rename `@differentiable(linear)` to `@differentiable(_linear)`.
- Make `@differentiable(reverse)` type lowering go through today's `@differentiable` code path. We will specialize it to reverse-mode in a follow-up patch.
ABI:
- Add `Forward` and `Reverse` to `FunctionMetadataDifferentiabilityKind`.
- Extend `TargetFunctionTypeFlags` by 1 bit to store the highest bit of differentiability kind (linear). Note that there is a 2-bit gap in `DifferentiabilityMask` which is reserved for `AsyncMask` and `ConcurrentMask`; `AsyncMask` is ABI-stable so we cannot change that.
_Differentiation module:
- Replace all occurrences of `@differentiable` with `@differentiable(reverse)`.
- Delete `_transpose(of:)`.
Resolves rdar://69980056.
Make `Optional` conditionally conform to `Differentiable` when the `Wrapped` type does.
`Optional.TangentVector` is a wrapper around `Wrapped.TangentVector?`.
Also, fix `Array.TangentVector.zeroTangentVectorInitializer`.
Resolves TF-1301.
`Differentiable` conformance derivation now supports
`Differentiable.zeroTangentVectorInitializer`.
There are two potential cases:
1. Memberwise derivation: done when `TangentVector` can be initialized memberwise.
2. `{ TangentVector.zero }` derivation: done as a fallback.
`zeroTangentVectorInitializer` is a closure that produces a zero tangent vector,
capturing minimal necessary information from `self`.
It is an instance property, unlike the static property `AdditiveArithmetic.zero`,
and should be used by the differentiation transform for correctness.
Remove `Differentiable.zeroTangentVectorInitializer` dummy default implementation.
Update stdlib `Differentiable` conformances and tests.
Clean up DerivedConformanceDifferentiable.cpp cruft.
Resolves TF-1007.
Progress towards TF-1008: differentiation correctness for projection operations.
The derivative wrt `self` should drop the last element from the incoming seed.
Example:
- Incoming seed: [1, 2, 3, 4]
- Derivative wrt `self`: [1, 2, 3]
- Derivative wrt appended element: 4
