[SDXL] Fix SDXL Inpaint when using from_single_file

[tolgacangoz]

What does this PR do?

Fixes #6001.
Previously, StableDiffusionXLInpaintPipeline with SDXL-refiner returned StableDiffusionXLImg2ImgPipeline wrongly when using the from_single_file method.

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@sayakpaul @DN6

[DN6]

Hi @StandardAI I opened a similar PR: #6147 to address this issue as well. There are quite a few extra checks in the convert_from_ckpt function and we're trying to simplify this function as much as possible moving forward.