[Wan] Fix VAE sampling mode in WanVideoToVideoPipeline

[tolgacangoz]

While integrating SkyReels-V2 models, I came across this: Major Wan-related repos including Wan-Video/Wan2.1, modelscope/DiffSynth-Studio, and SkyworkAI/SkyReels-V2 prefer sample_mode == "argmax" for the encoding's output of their VAEs. Also, at the other Wan pipelines in diffusers, too. Am I correct?
Also, fixes a typo.

Input Video
hiker.mp4
Previous	Current
wan-v2v.mp4	wan-v2v-fixed.mp4

I am unsure if there is supposed to be a visible fix 🤔.

Reproducer

#!pip uninstall diffusers -yq
#!pip install git+https://github.com/tolgacangoz/diffusers.git@fix-wanv2v-vae ftfy -q
import torch
from diffusers.utils import load_video, export_to_video
from diffusers import AutoencoderKLWan, WanVideoToVideoPipeline, UniPCMultistepScheduler

# Available models: Wan-AI/Wan2.1-T2V-14B-Diffusers, Wan-AI/Wan2.1-T2V-1.3B-Diffusers
model_id = "Wan-AI/Wan2.1-T2V-1.3B-Diffusers"
vae = AutoencoderKLWan.from_pretrained(
    model_id, subfolder="vae", torch_dtype=torch.float32
)
pipe = WanVideoToVideoPipeline.from_pretrained(
    model_id, vae=vae, torch_dtype=torch.bfloat16
)
flow_shift = 3.0  # 5.0 for 720P, 3.0 for 480P
pipe.scheduler = UniPCMultistepScheduler.from_config(
    pipe.scheduler.config, flow_shift=flow_shift
)
#pipe.enable_model_cpu_offload()
pipe = pipe.to('cuda')

prompt = "A robot standing on a mountain top. The sun is setting in the background"
negative_prompt = "Bright tones, overexposed, static, blurred details, subtitles, style, works, paintings, images, static, overall gray, worst quality, low quality, JPEG compression residue, ugly, incomplete, extra fingers, poorly drawn hands, poorly drawn faces, deformed, disfigured, misshapen limbs, fused fingers, still picture, messy background, three legs, many people in the background, walking backwards"
video = load_video(
    "https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/diffusers/hiker.mp4"
)
output = pipe(
    video=video,
    prompt=prompt,
    negative_prompt=negative_prompt,
    height=480,
    width=512,
    guidance_scale=7.0,
    strength=0.7,
    generator=torch.Generator(device="cuda").manual_seed(0),
).frames[0]

export_to_video(output, "wan-v2v.mp4", fps=16)

@DN6 @a-r-r-o-w

[a-r-r-o-w]

Looks correct to me because we use argmax in the other pipelines as well IIRC

[ukaprch]

In your "current" fix mp4 it appears that you have outliers "pixels" above / below the RGB threshold. I'm talking about those red spots and the light beam that moves across the video.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[tolgacangoz]

In your "current" fix mp4 it appears that you have outliers "pixels" above / below the RGB threshold. I'm talking about those red spots and the light beam that moves across the video.

When I complete SkyReels-V2 integration, I will return to this PR. It is almost done.