Fix black image output in QwenImageEdit docstring

[Praanjaljain29]

Fixes #13819

What does this PR do?

This fixes a bug in the QwenImageEditPipeline documentation snippet where users were getting a black image output.

Because the pipeline was initialized in torch.bfloat16, the AutoencoderKL (VAE) mathematically overflows during the decode step, resulting in NaN values that the image processor converts to black pixels.

This PR adds pipe.vae.to(torch.float32) to the docstring example to prevent the overflow and ensure proper image generation.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
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• Was this discussed/approved via a GitHub issue?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@yiyixuxu @patrickvonplaten