Fix Z-Image guidance scale threshold for CFG

Z-Image uses a different CFG formula than standard diffusers pipelines:
- Standard: pred = neg + guidance_scale * (pos - neg), where scale=1 means no CFG
- Z-Image: pred = pos + guidance_scale * (pos - neg), where scale=0 means no CFG

The do_classifier_free_guidance property was checking > 1, which prevented
CFG from being applied when guidance_scale was between 0 and 1. Changed
to > 0 to match Z-Image's CFG semantics.

Fixes huggingface#12905

Author: CalamitousFelicitousness

src/diffusers/pipelines/z_image/pipeline_z_image.py

@@ -276,7 +276,7 @@ def guidance_scale(self):
 
     @property
     def do_classifier_free_guidance(self):
-        return self._guidance_scale > 1
+        return self._guidance_scale > 0
 
     @property
     def joint_attention_kwargs(self):

src/diffusers/pipelines/z_image/pipeline_z_image_controlnet.py

@@ -365,7 +365,7 @@ def guidance_scale(self):
 
     @property
     def do_classifier_free_guidance(self):
-        return self._guidance_scale > 1
+        return self._guidance_scale > 0
 
     @property
     def joint_attention_kwargs(self):

src/diffusers/pipelines/z_image/pipeline_z_image_controlnet_inpaint.py

@@ -372,7 +372,7 @@ def guidance_scale(self):
 
     @property
     def do_classifier_free_guidance(self):
-        return self._guidance_scale > 1
+        return self._guidance_scale > 0
 
     @property
     def joint_attention_kwargs(self):

src/diffusers/pipelines/z_image/pipeline_z_image_img2img.py

@@ -347,7 +347,7 @@ def guidance_scale(self):
 
     @property
     def do_classifier_free_guidance(self):
-        return self._guidance_scale > 1
+        return self._guidance_scale > 0
 
     @property
     def joint_attention_kwargs(self):

src/diffusers/pipelines/z_image/pipeline_z_image_inpaint.py

@@ -462,7 +462,7 @@ def guidance_scale(self):
 
     @property
     def do_classifier_free_guidance(self):
-        return self._guidance_scale > 1
+        return self._guidance_scale > 0
 
     @property
     def joint_attention_kwargs(self):

src/diffusers/pipelines/z_image/pipeline_z_image_omni.py

@@ -339,7 +339,7 @@ def guidance_scale(self):
 
     @property
     def do_classifier_free_guidance(self):
-        return self._guidance_scale > 1
+        return self._guidance_scale > 0
 
     @property
     def joint_attention_kwargs(self):