fix: TensorRTStableDiffusionPipeline cannot set guidance_scale

[caiyueliang]

What does this PR do?

fix the bug when I use custom pipeline: TensorRTStableDiffusionPipeline:

Traceback (most recent call last):
  File "exchange_torch2trt.py", line 93, in <module>
    gen_image(pipe=pipe, prompt=prompt, negative_prompt=neg_prompt)
  File "exchange_torch2trt.py", line 72, in gen_image
    image = pipe(prompt, negative_prompt=negative_prompt).images[0]
  File "/usr/local/lib/python3.8/site-packages/torch/utils/_contextlib.py", line 115, in decorate_context
    return func(*args, **kwargs)
  File "/root/.cache/huggingface/modules/diffusers_modules/local/stable_diffusion_tensorrt_txt2img.py", line 885, in __call__
    self.guidance_scale = guidance_scale
  File "/usr/local/lib/python3.8/site-packages/diffusers/pipelines/pipeline_utils.py", line 622, in __setattr__
    super().__setattr__(name, value)
AttributeError: can't set attribute

The solution is to refer to the: #6326
And it has been verified that it can operate normally.

Fixes # (issue)

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[yiyixuxu]

thanks!

[HuggingFaceDocBuilderDev]

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