Fix prelu ref when a.ndim < 2 (#89809)
Fixes https://github.com/pytorch/pytorch/issues/89560
Previously the test case for "input is 1-D or scalar + weight is not scalar" did not exist; adding it introduced some failures:
- forward AD (fixed in this PR)
- vmap (filed https://github.com/pytorch/pytorch/issues/89895)
- ref/meta (fixed this PR, though this also regresses nvFuser support)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89809
Approved by: https://github.com/ngimel
