Make SDXL treat the "optional" components as really optional

[ppbrown]

What does this PR do?

This makes the "optional" text modules in StableDiffusionXLPipeline actually function as optional, instead of mandatory.
(There was already code present that suggests you can optionally skip modules: unfortunately, the code does not actually work that way, so this fixes the unfiled bug)

The patch can be verified by running the code mentioned in the target model in the README.md at
https://huggingface.co/opendiffusionai/sdxlone

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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• Did you write any new necessary tests?

Who can review?

Core library:

• Pipelines and pipeline callbacks: @yiyixuxu and @asomoza

[asomoza]

@ppbrown thanks for your contribution.

We usually only add additional code to the official pipelines and the core when there's a fully working and released model that it's also popular, to avoid adding more complexity and making it a huge library with unused models.

For what I can see in your model repo:

So, I created this version to experimentally prove the better way.

and in the other model there is this part:

This raw version of the model seems to work great with 3-5 word prompts... but then decays after that. So I'm working on a finetuned version.

both texts suggest that this is just and experiment that needs more finetuning, also it's not that popular right now.

I suggest that we add this as a community pipeline for the moment so people can test your model and we can see then how this goes for the community, also it would make it a lot faster to merge.