[functorch] fix vmapvjpvjp test for prelu (#84939)
Turns out this is just a composite compliance issue. Branching on if
something requires grad or not can lead to incorrect gradients if we
have a BatchedTensor wrapping a tensor that requires grad.
Test Plan:
- tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84939
Approved by: https://github.com/soulitzer
