sifier-Free Diffusion Guidance.
guidance_scale is defined as w of equation 2. of Imagen
Paper. Guidance scale is enabled by setting guidance_scale > 1. Higher guidance scale encourages to generate images that are closely linked to the text prompt,
usually at the expense of lower image quality.   Returns 
KandinskyPriorPipelineOutput or tuple
    Function invoked when using the prior pipeline for interpolation.  Examples:   Copied >>> from diffu