[Feat] Adds LongCat-AudioDiT pipeline

[RuixiangMa]

What does this PR do?

Adds LongCat-AudioDiT model support to diffusers.

Although LongCat-AudioDiT can be used for TTS-like generation, it is fundamentally a diffusion-based audio generation model (text conditioning + iterative latent denoising + VAE decoding) rather than a conventional autoregressive TTS model, so i think it fits naturally into diffusers.

Test

import soundfile as sf
import torch
from diffusers import LongCatAudioDiTPipeline

pipeline = LongCatAudioDiTPipeline.from_pretrained(
    "meituan-longcat/LongCat-AudioDiT-1B",
    torch_dtype=torch.float16,
)
pipeline = pipeline.to("cuda")

audio = pipeline(
    prompt="A calm ocean wave ambience with soft wind in the background.",
    audio_end_in_s=5.0,
    num_inference_steps=16,
    guidance_scale=4.0,
    output_type="pt",
).audios

output = audio[0, 0].float().cpu().numpy()
sf.write("longcat.wav", output, pipeline.sample_rate)

Result

longcat.wav

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