[AutoDiff] Fix Array.append(_:) pullback.

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

Author: dan-zheng

stdlib/public/Differentiation/ArrayDifferentiation.swift

@@ -222,8 +222,11 @@ extension Array where Element: Differentiable {
     value: Void, pullback: (inout TangentVector) -> Element.TangentVector
   ) {
     let appendedElementIndex = count
-    defer { append(element) }
-    return ((), { dself in dself.base[appendedElementIndex] })
+    append(element)
+    return ((), { v in
+      defer { v.base.removeLast() }
+      return v.base[appendedElementIndex]
+    })
   }
 
   @usableFromInline

test/AutoDiff/validation-test/array.swift

@@ -372,15 +372,28 @@ ArrayAutoDiffTests.test("Array.+=") {
 }
 
 ArrayAutoDiffTests.test("Array.append") {
+  func appending(_ array: [Float], _ element: Float) -> [Float] {
+    var result = array
+    result.append(element)
+    return result
+  }
+  do {
+    let v = FloatArrayTan([1, 2, 3, 4])
+    expectEqual((.init([1, 2, 3]), 4),
+                pullback(at: [0, 0, 0], 0, in: appending)(v))
+  }
+
   func identity(_ array: [Float]) -> [Float] {
     var results: [Float] = []
     for i in withoutDerivative(at: array.indices) {
       results.append(array[i])
     }
     return results
   }
-  let v = FloatArrayTan([4, -5, 6])
-  expectEqual(v, pullback(at: [1, 2, 3], in: identity)(v))
+  do {
+    let v = FloatArrayTan([4, -5, 6])
+    expectEqual(v, pullback(at: [1, 2, 3], in: identity)(v))
+  }
 }
 
 ArrayAutoDiffTests.test("Array.init(repeating:count:)") {