LongCat Image pipeline: Allow offloading/quantization of text_encoder component

[Yahweasel]

What does this PR do?

Remove the attempt to move text_encoder to the same device as the rest of the pipeline in pipeline_longcat_image.py. The text encoder may not be movable (VRAM limitations), and more to the point, it's the user's prerogative where they load things, not the pipeline's. Instead, simply moves the tensor that text_encoder generates.

Also (and my main motivation) fixes quantization of the LongCat Image pipeline using bitsandbytes, which previously gave this exception:

  File "/home/yahweasel/ComfyUI/venv/lib/python3.12/site-packages/diffusers/pipelines/longcat_image/pipeline_longcat_image.py", line 539, in __call__
    prompt = self.rewire_prompt(prompt, device)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/yahweasel/ComfyUI/venv/lib/python3.12/site-packages/diffusers/pipelines/longcat_image/pipeline_longcat_image.py", line 265, in rewire_prompt
    self.text_encoder.to(device)
  File "/home/yahweasel/ComfyUI/venv/lib/python3.12/site-packages/transformers/modeling_utils.py", line 4328, in to
    raise ValueError(
ValueError: `.to` is not supported for `8-bit` bitsandbytes models. Please use the model as it is, since the model has already been set to the correct devices and casted to the correct `dtype`.

Here's a simple example with bitsandbytes, which now works:

qc = diffusers.PipelineQuantizationConfig(quant_backend="bitsandbytes-8bit", quant_kwargs={"load_in_8bit": True})
pipe = diffusers.LongCatImagePipeline("meituan-longcat/LongCat-Image", quantization_config=qc)
pipe("a man").images[0]

I did not submit an issue for this error, because the fix was trivial.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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  (There is no relevant documentation, as this is simply a tensor location)
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  (Are new tests necessary for this?)

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