Fix access to unitialized memory in VSX vector functions (#89833)
This results in e.g. failures in TestNNDeviceTypeCPU.test_groupnorm_nhwc_cpu_float32
So simply initialize the stack array with zeroes as expected and done in other implementations
Fixes #32502
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89833
Approved by: https://github.com/ezyang
