[AutoDiff] Add more loop differentiation negative testcases for TF-933. (#27973)

Loop differentiation produces incorrect results when the reduction accumulation
variable is not initialized with an active parameter. TF-933 tracks this issue.

This patch is a follow-up to #27796, adding
negative test cases for while and repeat-while loops.

Author: dan-zheng

test/AutoDiff/control_flow.swift

@@ -556,6 +556,22 @@ ControlFlowTests.test("Loops") {
   expectEqual((8, 12), valueWithGradient(at: 2, in: while_loop))
   expectEqual((27, 27), valueWithGradient(at: 3, in: while_loop))
 
+  func while_loop_nonactive_initial_value(_ x: Float) -> Float {
+    var result: Float = 1
+    var i = 0
+    while i < 2 {
+      result = result * x
+      i += 1
+    }
+    return result
+  }
+  // TODO(TF-933): Fix incorrect derivatives when `var result` is not initially
+  // assigned to `x`.
+  // expectEqual((4, 4), valueWithGradient(at: 2, in: while_loop_nonactive_initial_value))
+  // expectEqual((9, 6), valueWithGradient(at: 3, in: while_loop_nonactive_initial_value))
+  expectEqual((4, 2), valueWithGradient(at: 2, in: while_loop_nonactive_initial_value))
+  expectEqual((9, 3), valueWithGradient(at: 3, in: while_loop_nonactive_initial_value))
+
   func repeat_while_loop(_ x: Float) -> Float {
     var result = x
     var i = 0
@@ -572,6 +588,22 @@ ControlFlowTests.test("Loops") {
   expectEqual((8, 18), valueWithGradient(at: 2, in: repeat_while_loop))
   expectEqual((27, 36), valueWithGradient(at: 3, in: repeat_while_loop))
 
+  func repeat_while_loop_nonactive_initial_value(_ x: Float) -> Float {
+    var result: Float = 1
+    var i = 0
+    repeat {
+      result = result * x
+      i += 1
+    } while i < 2
+    return result
+  }
+  // TODO(TF-584, TF-933): Fix incorrect derivatives when `var result` is not
+  // initially assigned to `x`.
+  // expectEqual((4, 4), valueWithGradient(at: 2, in: repeat_while_loop_nonactive_initial_value))
+  // expectEqual((9, 6), valueWithGradient(at: 3, in: repeat_while_loop_nonactive_initial_value))
+  expectEqual((4, 3), valueWithGradient(at: 2, in: repeat_while_loop_nonactive_initial_value))
+  expectEqual((9, 4), valueWithGradient(at: 3, in: repeat_while_loop_nonactive_initial_value))
+
   func loop_continue(_ x: Float) -> Float {
     var result = x
     for i in 1..<10 {