Add stochastic sampling to FlowMatchEulerDiscreteScheduler

[apolinario]

What does this PR do?

This PR adds stochastic sampling to FlowMatchEulerDiscreteScheduler based on Lightricks/LTX-Video@b1aeddd ltx_video/schedulers/rf.py, which was added with th release of 0.9.6-distilled. I decoupled the next and current sigma to try to get closer to the rf.py implementation of the stochastic sampling, but a second pair of eyes on this would be great.

To try it:

import torch
from diffusers import LTXVideoTransformer3DModel, FlowMatchEulerDiscreteScheduler, LTXPipeline
from diffusers.utils import export_to_video

transformer = LTXVideoTransformer3DModel.from_pretrained(
    "multimodalart/ltxv-2b-0.9.6-distilled",
    subfolder="transformer",
    torch_dtype=torch.bfloat16,
    variant="bf16"
)

scheduler = FlowMatchEulerDiscreteScheduler.from_pretrained(
    "multimodalart/ltxv-2b-0.9.6-distilled",
    subfolder="scheduler"
)

pipe = LTXPipeline.from_pretrained(
    "Lightricks/LTX-Video-0.9.5",
    transformer=transformer,
    scheduler=scheduler, #add or remove the scheduler to see the difference
    torch_dtype=torch.bfloat16,
)
pipe.to("cuda")

prompt = "A woman eating a burger"
negative_prompt = "worst quality, inconsistent motion, blurry, jittery, distorted"
generator = torch.Generator(device="cuda").manual_seed(42)
video = pipe(
    prompt=prompt,
    negative_prompt=negative_prompt,
    width=1216,
    height=704,
    num_frames=121,
    num_inference_steps=8,
    guidance_scale=1,
    generator=generator
).frames[0]

export_to_video(video, "distilled_scheduler.mp4", fps=24)

Who can review?

@yiyixuxu

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[apolinario]

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[yiyixuxu]

thanks @apolinario

[yiyixuxu]

I think just have this in config is enough no?

[apolinario]

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[yiyixuxu]

Suggested change

stochastic_sampling: Optional[bool] = None,

[yiyixuxu]

Suggested change

	stochastic_sampling (`bool`, *optional*):
	Whether to use stochastic sampling. If None, defaults to the value set in the scheduler's config.

[yiyixuxu]

@apolinario
here it seems to reversed, no?
before:
dt = (per_token_sigmas - lower_sigmas)[..., None]

now:
dt = ower_sigmas - per_token_sigmas