Fix PriorTransformer Group Offloading Bug (#14695)

[Stable unCLIP] Fix Stable unCLIP group offloading tests

`PriorTransformer` holds `clip_mean` / `clip_std` as parameters of the model
itself rather than of a submodule, so group offloading gathers them into the
"unmatched group" whose hook wraps the top-level `forward`. They are therefore
onloaded only for the duration of `forward`, while `post_process_latents` runs
after the denoising loop and saw them back on the offload device:

    RuntimeError: Expected all tensors to be on the same device, but found at
    least two devices, cuda:0 and cpu!

Move them to the latents' device explicitly.

This also fixes the same failure in the three Kandinsky prior pipelines, whose
strict xfail markers now XPASS, so remove them along with the overrides that
existed only to carry them. Their `PIPELINE_GROUP_OFFLOAD_XFAIL_REASON` was a
misdiagnosis: the model-level parameters are onloaded for `forward`, and the
pipeline-level test was failing on `post_process_latents` like the others.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>