Fall back to PyTorch attention when FA3 is unsupported

app.py

@@ -50,7 +50,10 @@ pipe.fuse_lora(adapter_names=["angles"], lora_scale=1.25)
 pipe.unload_lora_weights()
 
 pipe.transformer.__class__ = QwenImageTransformer2DModel
-pipe.transformer.set_attn_processor(QwenDoubleStreamAttnProcessorFA3())
+try:
+    pipe.transformer.set_attn_processor(QwenDoubleStreamAttnProcessorFA3())
+except ImportError as error:
+    print(f"FlashAttention-3 is unavailable; using PyTorch attention instead: {error}")
 
 optimize_pipeline_(
     pipe,