Add comments to SDXL Turbo example (#512)

* Add comments to SDXL Turbo example

* Address PR comments

---------

Co-authored-by: Ubuntu <[email protected]>

06_gpu_and_ml/stable_diffusion/stable_diffusion_xl_turbo.py

@@ -1,12 +1,39 @@
+# ---
+# output-directory: "/tmp/stable-diffusion-xl-turbo"
+# args: []
+# runtimes: ["runc", "gvisor"]
+# ---
+# # Stable Diffusion XL Turbo Image-to-image
+#
+# This example is similar to the [Stable Diffusion XL](/docs/examples/stable_diffusion_xl)
+# example, but it's a distilled model trained for real-time synthesis and is image-to-image. Learn more about it [here](https://stability.ai/news/stability-ai-sdxl-turbo).
+#
+# Input prompt:
+# `dog wizard, gandalf, lord of the rings, detailed, fantasy, cute, adorable, Pixar, Disney, 8k`
+#
+# Input             |  Output
+# :-------------------------:|:-------------------------:
+# ![](./stable_diffusion_turbo_input.png)  |  ![](./stable_diffusion_turbo_output.png)
+
+# ## Basic setup
+
 from pathlib import Path
 
 from modal import Image, Stub, gpu, method
 
+# ## Define a container image
+
 
 def download_models():
     from huggingface_hub import snapshot_download
 
-    ignore = ["*.bin", "*.onnx_data", "*/diffusion_pytorch_model.safetensors"]
+    # Ignore files that we don't need to speed up download time.
+    ignore = [
+        "*.bin",
+        "*.onnx_data",
+        "*/diffusion_pytorch_model.safetensors",
+    ]
+
     snapshot_download("stabilityai/sdxl-turbo", ignore_patterns=ignore)
 
 
@@ -15,15 +42,23 @@ def download_models():
     .pip_install(
         "Pillow~=10.1.0",
         "diffusers~=0.24",
-        "transformers~=4.35",
-        "accelerate~=0.25",
-        "safetensors~=0.4",
+        "transformers~=4.35",  # This is needed for `import torch`
+        "accelerate~=0.25",  # Allows `device_map="auto"``, which allows computation of optimized device_map
+        "safetensors~=0.4",  # Enables safetensor format as opposed to using unsafe pickle format
     )
     .run_function(download_models)
 )
 
 stub = Stub("stable-diffusion-xl-turbo", image=image)
 
+# ## Load model and run inference
+#
+# The container lifecycle [`__enter__` function](https://modal.com/docs/guide/lifecycle-functions#container-lifecycle-beta)
+# loads the model at startup. Then, we evaluate it in the `inference` function.
+#
+# To avoid excessive cold-starts, we set the idle timeout to 240 seconds, meaning once a GPU has loaded the model it will stay
+# online for 4 minutes before spinning down. This can be adjusted for cost/experience trade-offs.
+
 
 @stub.cls(gpu=gpu.A10G(), container_idle_timeout=240)
 class Model:
@@ -48,11 +83,17 @@ def inference(self, image_bytes, prompt):
         init_image = load_image(Image.open(BytesIO(image_bytes))).resize(
             (512, 512)
         )
+        num_inference_steps = 4
+        strength = 0.9
+        # "When using SDXL-Turbo for image-to-image generation, make sure that num_inference_steps * strength is larger or equal to 1"
+        # See: https://huggingface.co/stabilityai/sdxl-turbo
+        assert num_inference_steps * strength >= 1
+
         image = self.pipe(
             prompt,
             image=init_image,
-            num_inference_steps=4,
-            strength=0.9,
+            num_inference_steps=num_inference_steps,
+            strength=strength,
             guidance_scale=0.0,
         ).images[0]
 
@@ -80,3 +121,11 @@ def main(
     print(f"Saving it to {output_path}")
     with open(output_path, "wb") as f:
         f.write(output_image_bytes)
+
+
+# ## Running the model
+#
+# We can run the model with different parameters using the following command,
+# ```
+# modal run stable_diffusion_xl_turbo.py --prompt="harry potter, glasses, wizard" --image-path="dog.png"
+# ```