demo_lowvram.py fails due to image_lr being None

[Lobo75]

Looks like from the comments image_lr in pipeline_demofustion_sdxl.py is optional but it's not properly handled when lowvram is set to true.

I'm not sure what a proper value would be for image_lr in pipeline_demofustion_sdxl.py when called from demo_lowvram.py but since it's optional the following change worked for me to allow the demo to run.

pipeline_demofusion_sdxl.py line 898:

    if self.lowvram:
        self.vae.cpu()
        self.unet.cpu()
        self.text_encoder.to(device)
        self.text_encoder_2.to(device)

        if (image_lr is not None):
          image_lr.cpu()

[RuoyiDu]

@Lobo75 Thanks for pointing this out! We will fix it.