Fixed SDXL single file loading to use the correct requested pipeline class (#4142)

Using pipeline_class argument to instantiate the correct pipeline when loading SDXL models from single files

src/diffusers/pipelines/stable_diffusion/convert_from_ckpt.py

@@ -1536,7 +1536,7 @@ def download_from_original_stable_diffusion_ckpt(
                 checkpoint, config_name, prefix="conditioner.embedders.1.model.", has_projection=True, **config_kwargs
             )
 
-            pipe = StableDiffusionXLPipeline(
+            pipe = pipeline_class(
                 vae=vae,
                 text_encoder=text_encoder,
                 tokenizer=tokenizer,