[fix] VaeImageProcessor.resize method can take list as input

[Net-Mist]

What does this PR do?

Hello !

When working on Flux-Kontext, I discovered a silent bug regarding the resizing of the image, when calling the pipeline with several images.

• The __call__ method of the FluxKontextPipeline class takes an image of type Optional[PipelineImageInput]. This variable can be a list in case of batch size > 1
• then the resize method of the VaeImageProcessor class is called (image = self.image_processor.resize(image, image_height, image_width)). But this method does not support lists as input. Due to the way this method is written (overriding the image variable and then returning it), it doesn't resize the image and return it as-is.

In this fix, I corrected this by changing a minimal number of lines of code to handle lists. However, please let me know if you'd prefer that I also add additional input checks or safeguards.

Fixes # (issue)

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[Net-Mist]

@yiyixuxu @asomoza
If I'm not mistaken, you are the 2 most relevant people for this PR. Else, sorry for the noise, and please redirect me to the right people