Stable Diffusion Turbo Support

[edwin0cheng]

This PR added following features in the stable-diffusion example:

• --sd-version turbo
• --guidance-scale for controlling guidance scale, which SD Turbo need to set it to 0 for the correct result.

you can try this by:

 cargo run --release --example stable-diffusion --features "cuda,cudnn" -- --prompt "A cinematic shot of a baby racoon wearing an intricate italian priest robe." --use-f16 --sd-version turbo --n-steps 1 --width 512 --height 512 --guidance-scale 0

[LaurentMazare]

Thanks for the PR, I made some small tweaks so that the default values for n_steps and guidance_scale are automatically inferred when using the turbo version so hopefully no need for the user to set these flags manually.