[Bugfix] Fix stable diffusion3 compatibility error

[iwzbi]

Purpose

fix #636

Test Plan

import torch

from vllm_omni import Omni
generator = torch.Generator("cuda").manual_seed(42)
model_name = "/model/stable-diffusion-3-medium-diffusers/"
m = Omni(model=model_name, dtype=torch.half)
images = m.generate(
    "A cat holding a sign that says hello world",
    negative_prompt="",
    height=height,
    width=width,
    dtype=torch.half,
    num_inference_steps=28,
    guidance_scale=7,
    generator=generator,
    num_outputs_per_prompt=1,
)

Test Result

[hsliuustc0106]

lgtm