[Flux] Improve true cfg condition (#10539)

* improve flux true cfg condition

* add test

src/diffusers/pipelines/flux/pipeline_flux.py

@@ -790,7 +790,10 @@ class FluxPipeline(
         lora_scale = (
             self.joint_attention_kwargs.get("scale", None) if self.joint_attention_kwargs is not None else None
         )
-        do_true_cfg = true_cfg_scale > 1 and negative_prompt is not None
+        has_neg_prompt = negative_prompt is not None or (
+            negative_prompt_embeds is not None and negative_pooled_prompt_embeds is not None
+        )
+        do_true_cfg = true_cfg_scale > 1 and has_neg_prompt
         (
             prompt_embeds,
             pooled_prompt_embeds,

tests/pipelines/flux/test_pipeline_flux.py

@@ -209,6 +209,17 @@ class FluxPipelineFastTests(unittest.TestCase, PipelineTesterMixin, FluxIPAdapte
             output_height, output_width, _ = image.shape
             assert (output_height, output_width) == (expected_height, expected_width)
 
+    def test_flux_true_cfg(self):
+        pipe = self.pipeline_class(**self.get_dummy_components()).to(torch_device)
+        inputs = self.get_dummy_inputs(torch_device)
+        inputs.pop("generator")
+
+        no_true_cfg_out = pipe(**inputs, generator=torch.manual_seed(0)).images[0]
+        inputs["negative_prompt"] = "bad quality"
+        inputs["true_cfg_scale"] = 2.0
+        true_cfg_out = pipe(**inputs, generator=torch.manual_seed(0)).images[0]
+        assert not np.allclose(no_true_cfg_out, true_cfg_out)
+
 
 @nightly
 @require_big_gpu_with_torch_cuda