[AutoDiff] Add loop differentiation negative testcases. (#27796)
Loop differentiation produces incorrect results when the reduction accumulation
variable is not initialized with an active parameter, e.g. when using
`var result = 1` instead of `var result = x`.
```
func for_loop_nonactive_initial_value(_ x: Float) -> Float {
var result: Float = 1
for _ in 0..<2 {
result = result * x
}
return result
}
print(valueWithGradient(at: 3, in: for_loop_nonactive_initial_value))
// Actual: (value: 9.0, gradient: 3.0)
// Expected: (value: 9.0, gradient: 6.0)
```
TF-933 tracks this issue. This patch add negative testcases (currently failing).
Workarounds are to rewrite using `var result = x` or by using
`Array.differentiableReduce`.
