Feat: add dtype selection to PixArt

[SLAPaper]

add dtype selection to pixart loading so that it won't conflicting with fp8 mode of ComfyUI, which is incompatible with pixart models mentioned in #9

[SLAPaper]

type annotations removed

[city96]

Thanks, looks good now.

I thought about it more and I might be able to get FP8 support working for just the PixArt model. I'll take a shot at it tonight or tomorrow, then merge this if that doesn't pan out.

[city96]

Alright, I got started on proper FP8 support on this branch. I think it's doable, though I don't have much free time these days.

In the meantime I just added a quick workaround so the current code still works when launched with --fp8_e5m2-unet - it'll just falls back to fp16 or whatever the default dtype is. Can you test if this works for you?

[SLAPaper]

Fine, I'll test it afterwards

[SLAPaper]

The quick fix works fine, thanks!