[Single File] Add GGUF support for LTX

[DN6]

What does this PR do?

Some GGUF checkpoints remove the model.diffusion_model prefix from the original LTX file. This PR accounts for that change.

Fixes # (issue)

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[HuggingFaceDocBuilderDev]

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[sayakpaul]

Have tested with the following and it's working like a charm!

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

ckpt_path = (
    "https://huggingface.co/city96/LTX-Video-gguf/blob/main/ltx-video-2b-v0.9-Q3_K_S.gguf"
)
transformer = LTXVideoTransformer3DModel.from_single_file(
    ckpt_path,
    quantization_config=GGUFQuantizationConfig(compute_dtype=torch.bfloat16),
    torch_dtype=torch.bfloat16,
)
pipe = LTXPipeline.from_pretrained(
    "Lightricks/LTX-Video",
    transformer=transformer,
    generator=torch.manual_seed(0),
    torch_dtype=torch.bfloat16,
)
pipe.enable_model_cpu_offload()

prompt = "A woman with long brown hair and light skin smiles at another woman with long blonde hair. The woman with brown hair wears a black jacket and has a small, barely noticeable mole on her right cheek. The camera angle is a close-up, focused on the woman with brown hair's face. The lighting is warm and natural, likely from the setting sun, casting a soft glow on the scene. The scene appears to be real-life footage"
negative_prompt = "worst quality, inconsistent motion, blurry, jittery, distorted"

video = pipe(
    prompt=prompt,
    negative_prompt=negative_prompt,
    width=704,
    height=480,
    num_frames=161,
    num_inference_steps=50,
).frames[0]
export_to_video(video, "output_gguf_ltx.mp4", fps=24)

[a-r-r-o-w]

LGTM!

[sayakpaul]

@DN6 @a-r-r-o-w I have also added a mention of the GGUF single-file loading support in the LTXVideo docs. I hope that is okay.