fix pixart-sigma negative prompt handling (#8299)

* fix negative prompt

* fix

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Co-authored-by: yiyixuxu <yixu310@gmail,com>
Co-authored-by: YiYi Xu <yixu310@gmail.com>

src/diffusers/pipelines/pixart_alpha/pipeline_pixart_alpha.py

@@ -394,7 +394,7 @@ class PixArtAlphaPipeline(DiffusionPipeline):
 
         # get unconditional embeddings for classifier free guidance
         if do_classifier_free_guidance and negative_prompt_embeds is None:
-            uncond_tokens = [negative_prompt] * batch_size
+            uncond_tokens = [negative_prompt] * batch_size if isinstance(negative_prompt, str) else negative_prompt
             uncond_tokens = self._text_preprocessing(uncond_tokens, clean_caption=clean_caption)
             max_length = prompt_embeds.shape[1]
             uncond_input = self.tokenizer(

src/diffusers/pipelines/pixart_alpha/pipeline_pixart_sigma.py

@@ -320,7 +320,7 @@ class PixArtSigmaPipeline(DiffusionPipeline):
 
         # get unconditional embeddings for classifier free guidance
         if do_classifier_free_guidance and negative_prompt_embeds is None:
-            uncond_tokens = [negative_prompt] * batch_size
+            uncond_tokens = [negative_prompt] * batch_size if isinstance(negative_prompt, str) else negative_prompt
             uncond_tokens = self._text_preprocessing(uncond_tokens, clean_caption=clean_caption)
             max_length = prompt_embeds.shape[1]
             uncond_input = self.tokenizer(