Use the Euler Ancestral scheduler by default with SDXL

[freespirit]

This PR changes the default Scheduler for SDXL to Euler Ancestral as requested in #1508

[freespirit]

Hello!
I have an issue with the default Euler Configuration and I was hoping for some guidance with this PR.

The image I get after 17 steps looks like noise:

The default configuration for Euler Ancestral looks exactly the same as the JSON scheduler configuration so It seems that using it should just work.

The way I run the example is with metal support:

cargo run --example stable-diffusion --features=metal -- --prompt "A black cat by the window" --sd-version xl --n-steps 17

I was hoping for some guidelines before diving deeper into the Scheduler implementation and trying to debug the issue further 🤗