[Docs] Update TGATE in section optimization.

[WentianZhang-ML]

What does this PR do?

This PR fixes a bug in the code snippet of PixArt and improves the readability of the rest codes.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Who can review?

@stevhliu

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[stevhliu]

Thanks for the update! 🤗