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2ca26424 - adding `callback_on_step_end` for `StableDiffusionLDM3DPipeline` (#7149)

adding `callback_on_step_end` for `StableDiffusionLDM3DPipeline` (#7149) * refactor callback * run make style * add copy --------- Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>