[Tests and Docs] Add a test on serializing pipelines with components...

[Tests and Docs] Add a test on serializing pipelines with components containing fused LoRA modules (#4962)

* add: test to ensure pipelines can be saved with fused lora modules.

* add docs about serialization with fused lora.

* Apply suggestions from code review
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* Empty-Commit

* Update docs/source/en/training/lora.md
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

---------
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

docs/source/en/training/lora.md

@@ -34,7 +34,7 @@ the attention layers of a language model is sufficient to obtain good downstream
 
 [cloneofsimo](https://github.com/cloneofsimo) was the first to try out LoRA training for Stable Diffusion in the popular [lora](https://github.com/cloneofsimo/lora) GitHub repository. 🧨 Diffusers now supports finetuning with LoRA for [text-to-image generation](https://github.com/huggingface/diffusers/tree/main/examples/text_to_image#training-with-lora) and [DreamBooth](https://github.com/huggingface/diffusers/tree/main/examples/dreambooth#training-with-low-rank-adaptation-of-large-language-models-lora). This guide will show you how to do both.
 
-If you'd like to store or share your model with the community, login to your Hugging Face account (create [one](hf.co/join) if you don't have one already):
+If you'd like to store or share your model with the community, login to your Hugging Face account (create [one](https://hf.co/join) if you don't have one already):
 
 ```bash
 huggingface-cli login
@@ -321,7 +321,7 @@ pipe.fuse_lora()
 
 generator = torch.manual_seed(0)
 images_fusion = pipe(
-    "masterpiece, best quality, mountain", output_type="np", generator=generator, num_inference_steps=2
+    "masterpiece, best quality, mountain", generator=generator, num_inference_steps=2
 ).images
 
 # To work with a different `lora_scale`, first reverse the effects of `fuse_lora()`.
@@ -333,7 +333,48 @@ pipe.fuse_lora(lora_scale=0.5)
 
 generator = torch.manual_seed(0)
 images_fusion = pipe(
-    "masterpiece, best quality, mountain", output_type="np", generator=generator, num_inference_steps=2
+    "masterpiece, best quality, mountain", generator=generator, num_inference_steps=2
+).images
+```
+
+## Serializing pipelines with fused LoRA parameters
+
+Let's say you want to load the pipeline above that has its UNet fused with the LoRA parameters. You can easily do so by simply calling the `save_pretrained()` method on `pipe`. 
+
+After loading the LoRA parameters into a pipeline, if you want to serialize the pipeline such that the affected model components are already fused with the LoRA parameters, you should:
+
+* call `fuse_lora()` on the pipeline with the desired `lora_scale`, given you've already loaded the LoRA parameters into it.
+* call `save_pretrained()` on the pipeline. 
+
+Here is a complete example:
+
+```python
+from diffusers import DiffusionPipeline
+import torch 
+
+pipe = DiffusionPipeline.from_pretrained("stabilityai/stable-diffusion-xl-base-1.0", torch_dtype=torch.float16).to("cuda")
+lora_model_id = "hf-internal-testing/sdxl-1.0-lora"
+lora_filename = "sd_xl_offset_example-lora_1.0.safetensors"
+pipe.load_lora_weights(lora_model_id, weight_name=lora_filename)
+
+# First, fuse the LoRA parameters.
+pipe.fuse_lora()
+
+# Then save.
+pipe.save_pretrained("my-pipeline-with-fused-lora")
+```
+
+Now, you can load the pipeline and directly perform inference without having to load the LoRA parameters again:
+
+```python
+from diffusers import DiffusionPipeline
+import torch 
+
+pipe = DiffusionPipeline.from_pretrained("my-pipeline-with-fused-lora", torch_dtype=torch.float16).to("cuda")
+
+generator = torch.manual_seed(0)
+images_fusion = pipe(
+    "masterpiece, best quality, mountain", generator=generator, num_inference_steps=2
 ).images
 ```
 

tests/models/test_lora_layers.py

@@ -965,15 +965,11 @@ class SDXLLoraLoaderMixinTests(unittest.TestCase):
         pipeline_components, lora_components = self.get_dummy_components()
         sd_pipe = StableDiffusionXLPipeline(**pipeline_components)
         sd_pipe = sd_pipe.to(torch_device)
-        # sd_pipe.unet.set_default_attn_processor()
         sd_pipe.set_progress_bar_config(disable=None)
 
         _, _, pipeline_inputs = self.get_dummy_inputs(with_generator=False)
 
-        images = sd_pipe(
-            **pipeline_inputs,
-            generator=torch.manual_seed(0),
-        ).images
+        images = sd_pipe(**pipeline_inputs, generator=torch.manual_seed(0)).images
         images_slice = images[0, -3:, -3:, -1]
 
         # Emulate training.
@@ -993,9 +989,7 @@ class SDXLLoraLoaderMixinTests(unittest.TestCase):
             sd_pipe.load_lora_weights(os.path.join(tmpdirname, "pytorch_lora_weights.safetensors"))
 
         lora_images_scale_0_5 = sd_pipe(
-            **pipeline_inputs,
-            generator=torch.manual_seed(0),
-            cross_attention_kwargs={"scale": 0.5},
+            **pipeline_inputs, generator=torch.manual_seed(0), cross_attention_kwargs={"scale": 0.5}
         ).images
         lora_image_slice_scale_0_5 = lora_images_scale_0_5[0, -3:, -3:, -1]
 
@@ -1017,6 +1011,45 @@ class SDXLLoraLoaderMixinTests(unittest.TestCase):
             images_slice, lora_image_slice_scale_0_5, atol=1e-03
         ), "0.5 scale and no scale shouldn't match"
 
+    def test_save_load_fused_lora_modules(self):
+        pipeline_components, lora_components = self.get_dummy_components()
+        sd_pipe = StableDiffusionXLPipeline(**pipeline_components)
+        sd_pipe = sd_pipe.to(torch_device)
+        sd_pipe.set_progress_bar_config(disable=None)
+
+        _, _, pipeline_inputs = self.get_dummy_inputs(with_generator=False)
+
+        # Emulate training.
+        set_lora_weights(lora_components["unet_lora_layers"].parameters(), randn_weight=True, var=0.1)
+        set_lora_weights(lora_components["text_encoder_one_lora_layers"].parameters(), randn_weight=True, var=0.1)
+        set_lora_weights(lora_components["text_encoder_two_lora_layers"].parameters(), randn_weight=True, var=0.1)
+
+        with tempfile.TemporaryDirectory() as tmpdirname:
+            StableDiffusionXLPipeline.save_lora_weights(
+                save_directory=tmpdirname,
+                unet_lora_layers=lora_components["unet_lora_layers"],
+                text_encoder_lora_layers=lora_components["text_encoder_one_lora_layers"],
+                text_encoder_2_lora_layers=lora_components["text_encoder_two_lora_layers"],
+                safe_serialization=True,
+            )
+            self.assertTrue(os.path.isfile(os.path.join(tmpdirname, "pytorch_lora_weights.safetensors")))
+            sd_pipe.load_lora_weights(os.path.join(tmpdirname, "pytorch_lora_weights.safetensors"))
+
+        sd_pipe.fuse_lora()
+        lora_images_fusion = sd_pipe(**pipeline_inputs, generator=torch.manual_seed(0)).images
+        lora_image_slice_fusion = lora_images_fusion[0, -3:, -3:, -1]
+
+        with tempfile.TemporaryDirectory() as tmpdirname:
+            sd_pipe.save_pretrained(tmpdirname)
+            sd_pipe_loaded = StableDiffusionXLPipeline.from_pretrained(tmpdirname)
+
+        loaded_lora_images = sd_pipe_loaded(**pipeline_inputs, generator=torch.manual_seed(0)).images
+        loaded_lora_image_slice = loaded_lora_images[0, -3:, -3:, -1]
+
+        assert np.allclose(
+            lora_image_slice_fusion, loaded_lora_image_slice, atol=1e-03
+        ), "The pipeline was serialized with LoRA parameters fused inside of the respected modules. The loaded pipeline should yield proper outputs, henceforth."
+
 
 @slow
 @require_torch_gpu