[Model Card] standardize T2I Sdxl model card

[cosmo3769]

What does this PR do?

Related issue: #6891 for T2I Sdxl

Notebook link: test_model_card_template_t2i_sdxl.ipynb

Generated model card link: test-model-card-template-t2i-sdxl

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Who can review?

@sayakpaul

[sayakpaul]

Thanks so much!

Optional for this PR: Can we also take this opportunity to fix the type annotation of the save_model_card() method:

diffusers/examples/text_to_image/train_text_to_image_sdxl.py

Line 66 in 35fd84b

def save_model_card(

[HuggingFaceDocBuilderDev]

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[cosmo3769]

Optional for this PR: Can we also take this opportunity to fix the type annotation of the save_model_card() method:

With one PR, should I also fix the type annotation for t2i, t2i-lora as well?

[sayakpaul]

Yes sure. That will be awesome.