What does this PR do?

Added FromSingleFileMixin to this pipeline loader like the other FLUX pipelines. (So this pipeline can load from an all in one checkpoint file)

def load_img2img() -> FluxImg2ImgPipeline:
    """Load the pre-trained img2img pipeline."""
    pipeline = FluxImg2ImgPipeline.from_single_file(checkpoint_location, torch_dtype=torch.bfloat16
    ).to("cuda")
    return pipeline
pipeline = load_img2img()

can be used to test it if you have a flux checkpoint with other models mixxed into it, it works now after that change though from my tests

@yiyixuxu and @asomoza

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