[lora]: Fix Flux2 LoRA NaN test (#12714)

* up

* Update tests/lora/test_lora_layers_flux2.py
Co-authored-by: dg845 <58458699+dg845@users.noreply.github.com>

---------
Co-authored-by: dg845 <58458699+dg845@users.noreply.github.com>

tests/lora/test_lora_layers_flux2.py

@@ -15,17 +15,18 @@
 import sys
 import unittest
 
+import numpy as np
 import torch
 from transformers import AutoProcessor, Mistral3ForConditionalGeneration
 
 from diffusers import AutoencoderKLFlux2, FlowMatchEulerDiscreteScheduler, Flux2Pipeline, Flux2Transformer2DModel
 
-from ..testing_utils import floats_tensor, require_peft_backend
+from ..testing_utils import floats_tensor, require_peft_backend, torch_device
 
 
 sys.path.append(".")
 
-from .utils import PeftLoraLoaderMixinTests  # noqa: E402
+from .utils import PeftLoraLoaderMixinTests, check_if_lora_correctly_set  # noqa: E402
 
 
 @require_peft_backend
@@ -94,6 +95,46 @@ class Flux2LoRATests(unittest.TestCase, PeftLoraLoaderMixinTests):
 
         return noise, input_ids, pipeline_inputs
 
+    # Overriding because (1) text encoder LoRAs are not supported in Flux 2 and (2) because the Flux 2 single block
+    # QKV projections are always fused, it has no `to_q` param as expected by the original test.
+    def test_lora_fuse_nan(self):
+        components, _, denoiser_lora_config = self.get_dummy_components()
+        pipe = self.pipeline_class(**components)
+        pipe = pipe.to(torch_device)
+        pipe.set_progress_bar_config(disable=None)
+        _, _, inputs = self.get_dummy_inputs(with_generator=False)
+
+        denoiser = pipe.transformer if self.unet_kwargs is None else pipe.unet
+        denoiser.add_adapter(denoiser_lora_config, "adapter-1")
+        self.assertTrue(check_if_lora_correctly_set(denoiser), "Lora not correctly set in denoiser.")
+
+        # corrupt one LoRA weight with `inf` values
+        with torch.no_grad():
+            possible_tower_names = ["transformer_blocks", "single_transformer_blocks"]
+            filtered_tower_names = [
+                tower_name for tower_name in possible_tower_names if hasattr(pipe.transformer, tower_name)
+            ]
+            if len(filtered_tower_names) == 0:
+                reason = f"`pipe.transformer` didn't have any of the following attributes: {possible_tower_names}."
+                raise ValueError(reason)
+            for tower_name in filtered_tower_names:
+                transformer_tower = getattr(pipe.transformer, tower_name)
+                is_single = "single" in tower_name
+                if is_single:
+                    transformer_tower[0].attn.to_qkv_mlp_proj.lora_A["adapter-1"].weight += float("inf")
+                else:
+                    transformer_tower[0].attn.to_k.lora_A["adapter-1"].weight += float("inf")
+
+        # with `safe_fusing=True` we should see an Error
+        with self.assertRaises(ValueError):
+            pipe.fuse_lora(components=self.pipeline_class._lora_loadable_modules, safe_fusing=True)
+
+        # without we should not see an error, but every image will be black
+        pipe.fuse_lora(components=self.pipeline_class._lora_loadable_modules, safe_fusing=False)
+        out = pipe(**inputs)[0]
+
+        self.assertTrue(np.isnan(out).all())
+
     @unittest.skip("Not supported in Flux2.")
     def test_simple_inference_with_text_denoiser_block_scale(self):
         pass