Fix NameError in ZImageOmniPipeline when guidance_scale=0

[Ricardo-M-L]

What this PR does

ZImageOmniPipeline.__call__ defines negative_condition_siglip_embeds only inside the CFG guard:

https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/z_image/pipeline_z_image_omni.py#L581-L582

if self.do_classifier_free_guidance:
    negative_condition_siglip_embeds = [[se.clone() for se in batch] for batch in condition_siglip_embeds]

but a few lines later reads the same name unconditionally:

https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/z_image/pipeline_z_image_omni.py#L590-L593

condition_siglip_embeds = [None if sels == [] else sels + [None] for sels in condition_siglip_embeds]
negative_condition_siglip_embeds = [
    None if sels == [] else sels + [None] for sels in negative_condition_siglip_embeds
]

Why this is a real bug

do_classifier_free_guidance is self._guidance_scale > 0 (line 342), so any call with guidance_scale=0.0 leaves negative_condition_siglip_embeds unbound → NameError: name 'negative_condition_siglip_embeds' is not defined.

This is the exact configuration the pipeline's own EXAMPLE_DOC_STRING uses:

https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/z_image/pipeline_z_image_omni.py#L51-L58

>>> image = pipe(
...     prompt,
...     height=1024,
...     width=1024,
...     num_inference_steps=9,
...     guidance_scale=0.0,
...     generator=torch.Generator("cuda").manual_seed(42),
... ).images[0]

i.e. running the documented Z-Image-Turbo snippet crashes. The downstream consumption (line 648) is already guarded by if apply_cfg:, so the negative reshape only needs to fire when CFG is on.

Fix

Wrap the negative reshape in the same do_classifier_free_guidance check — matching the symmetric treatment of negative_condition_latents, which is only defined and only consumed when CFG is on.

 condition_siglip_embeds = [None if sels == [] else sels + [None] for sels in condition_siglip_embeds]
-negative_condition_siglip_embeds = [
-    None if sels == [] else sels + [None] for sels in negative_condition_siglip_embeds
-]
+if self.do_classifier_free_guidance:
+    negative_condition_siglip_embeds = [
+        None if sels == [] else sels + [None] for sels in negative_condition_siglip_embeds
+    ]

Before submitting

• Did you read the contributor guideline?
• Was this discussed/approved via a Github issue or the forum? N/A — crashes the pipeline's own doc example.
• Did you make sure to update the documentation with your changes? N/A.
• Did you write any new necessary tests? N/A — restores behavior matching the doc example.

Who can review?

@yiyixuxu @sayakpaul

[yiyixuxu]

thanks!

[HuggingFaceDocBuilderDev]

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[Ricardo-M-L]

Friendly ping — this PR has been approved. Is there anything else needed before merging? Happy to make any requested changes.