[Flax SDXL] fix zero out sdxl (huggingface#5203)

Author: patrickvonplaten

src/diffusers/pipelines/stable_diffusion_xl/pipeline_flax_stable_diffusion_xl.py

@@ -188,9 +188,10 @@ def _generate(
         # Get unconditional embeddings
         batch_size = prompt_embeds.shape[0]
         if neg_prompt_ids is None:
-            neg_prompt_ids = self.prepare_inputs([""] * batch_size)
-
-        neg_prompt_embeds, negative_pooled_embeds = self.get_embeddings(neg_prompt_ids, params)
+            neg_prompt_embeds = jnp.zeros_like(prompt_embeds)
+            negative_pooled_embeds = jnp.zeros_like(pooled_embeds)
+        else:
+            neg_prompt_embeds, negative_pooled_embeds = self.get_embeddings(neg_prompt_ids, params)
 
         add_time_ids = self._get_add_time_ids(
             (height, width), (0, 0), (height, width), prompt_embeds.shape[0], dtype=prompt_embeds.dtype