pick seed <= u32::MAX when using metal

[rgbkrk]

Problem

Running the stable diffusion example with Metal results in an error with the seed (possibly) being out of range.

$ cargo run --example stable-diffusion --release --features=metal -- --prompt "a cosmonaut on a horse (hd, realistic, high-def)" --sd-version turbo
   Compiling candle-transformers v0.9.1 (/Users/kylekelley/code/src/github.com/huggingface/candle/candle-transformers)
   Compiling candle-examples v0.9.1 (/Users/kylekelley/code/src/github.com/huggingface/candle/candle-examples)
    Finished `release` profile [optimized] target(s) in 13.34s
     Running `target/release/examples/stable-diffusion --prompt 'a cosmonaut on a horse (hd, realistic, high-def)' --sd-version turbo`
Using seed 14094623835642992148
Error: Metal error Metal seed must be less than or equal to u32::MAX

Caused by:
    Metal seed must be less than or equal to u32::MAX

The user can manually set --seed but we can make this easier.

Solution

Generate the seed up to u32::MAX when metal is enabled.

[greenrazer]

Thanks for the contribution!

[rgbkrk]

Addressed review feedback. I had to add cfg_if as a dependency as well.

[greenrazer]

Thanks for implementing this! I mistakenly thought cfg_if was in the standard library. To avoid adding external dependencies, I’ll need to roll back these most recent changes before merging. Really appreciate your work here, my mistake for not checking the dependency requirements first.

[rgbkrk]

No problem, thanks for maintaining the package.