add Instant id sdxl image2image pipeline

[linoytsaban]

Add an image to image pipeline version of instantid stable diffusion xl pipeline
for those who'd like to include an init image to be noised together with the generation.
Modeled from:

• https://github.com/huggingface/diffusers/blob/main/src/diffusers/examples/community/pipeline_stable_diffusion_xl_instantid.py
• https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/controlnet/pipeline_controlnet_sd_xl_img2img.py

• adds a check for insightface package to refer users to installations required
  cc: @apolinario

[HuggingFaceDocBuilderDev]

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[apolinario]

when we get the tests to pass we can merge i think

[linoytsaban]

what's failing the tests now is checking if insightface is installed - since it's imported but not used, it cases a style error.
differently than bitsandbytes (where we check if the package is installed for 8bitAdam- here it doesn't seem right to throw an import error if insightface is not installed as it's not a mandatory(or used at all) in the pipeline (only at inference for the face embeds)

[sayakpaul]

Great work!