MPS: scheduler indices must be in the same device as the indexed tensor

[pcuenca]

Continuation of #239, but specific for mps. /cc @FahimF. Being fixed in #450.

[FahimF]

Since #450 is merged to the main branch, I installed the code from the main branch and ran a bunch of tests on all the Stable Diffusion pipelines and schedulers that I currently use under mps. I can confirm that all of them work correctly and that there are no issues 😄

Here's what I can confirm works:
Pipelines: StableDiffusionPipeline and StableDiffusionImg2ImgPipeline
Schedulers: LMSDiscreteScheduler, PNDMScheduler, and DDIMScheduler