Fix vae load with --cpu-vae with AMD GPU

[N00byKing]

When running on AMD and using --cpu-vae, amd_min_version later in the code will cause a check on device, leading to Expected a cuda device, but got: cpu. is_amd does not catch this because the actual generation is happening on an AMD GPU.

This change mitigates this by always returning float32 if vae is done on CPU.
There may be cleaner ways to fix this; I am open to opinions on the matter.

[comfyanonymous]

This is a cleaner way of fixing this: #9754