Fix faulty, vectorized `pow` function on VSX (#82646)
This fixes the remaining bug introduced by the VSX optimized code in https://github.com/pytorch/pytorch/pull/41541
Followup to https://github.com/pytorch/pytorch/pull/59382
### Description
The code currently returns wrong results on POWER9LE making e.g. the `test_binary_ufuncs` fail.
### Testing
Build and ran tests on PPC
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82646
Approved by: https://github.com/ezyang
