[docs] Adapt a model (#3326)

* first draft

* apply feedback

* conv_in.weight thrown away

docs/source/en/_toctree.yml

@@ -62,6 +62,8 @@
       title: Overview
     - local: training/create_dataset
       title: Create a dataset for training
+    - local: training/adapt_a_model
+      title: Adapt a model to a new task
     - local: training/unconditional_training
       title: Unconditional image generation
     - local: training/text_inversion

docs/source/en/training/adapt_a_model.mdx

@@ -0,0 +1,42 @@
+# Adapt a model to a new task
+
+Many diffusion systems share the same components, allowing you to adapt a pretrained model for one task to an entirely different task.
+
+This guide will show you how to adapt a pretrained text-to-image model for inpainting by initializing and modifying the architecture of a pretrained [`UNet2DConditionModel`].
+
+## Configure UNet2DConditionModel parameters
+
+A [`UNet2DConditionModel`] by default accepts 4 channels in the [input sample](https://huggingface.co/docs/diffusers/v0.16.0/en/api/models#diffusers.UNet2DConditionModel.in_channels). For example, load a pretrained text-to-image model like [`runwayml/stable-diffusion-v1-5`](https://huggingface.co/runwayml/stable-diffusion-v1-5) and take a look at the number of `in_channels`:
+
+```py
+from diffusers import StableDiffusionPipeline
+
+pipeline = StableDiffusionPipeline.from_pretrained("runwayml/stable-diffusion-v1-5")
+pipeline.unet.config["in_channels"]
+4
+```
+
+Inpainting requires 9 channels in the input sample. You can check this value in a pretrained inpainting model like [`runwayml/stable-diffusion-inpainting`](https://huggingface.co/runwayml/stable-diffusion-inpainting):
+
+```py
+from diffusers import StableDiffusionPipeline
+
+pipeline = StableDiffusionPipeline.from_pretrained("runwayml/stable-diffusion-inpainting")
+pipeline.unet.config["in_channels"]
+9
+```
+
+To adapt your text-to-image model for inpainting, you'll need to change the number of `in_channels` from 4 to 9. 
+
+Initialize a [`UNet2DConditionModel`] with the pretrained text-to-image model weights, and change `in_channels` to 9. Changing the number of `in_channels` means you need to set `ignore_mismatched_sizes=True` and `low_cpu_mem_usage=False` to avoid a size mismatch error because the shape is different now.
+
+```py
+from diffusers import UNet2DConditionModel
+
+model_id = "runwayml/stable-diffusion-v1-5"
+unet = UNet2DConditionModel.from_pretrained(
+    model_id, subfolder="unet", in_channels=9, low_cpu_mem_usage=False, ignore_mismatched_sizes=True
+)
+```
+
+The pretrained weights of the other components from the text-to-image model are initialized from their checkpoints, but the input channel weights (`conv_in.weight`) of the `unet` are randomly initialized. It is important to finetune the model for inpainting because otherwise the model returns noise.