[Docs] Add guide for AutoModel with custom code (#13099)

update

Author: DN6

docs/source/en/using-diffusers/automodel.md

@@ -29,8 +29,31 @@ text_encoder = AutoModel.from_pretrained(
 )
 ```
 
+## Custom models
+
 [`AutoModel`] also loads models from the [Hub](https://huggingface.co/models) that aren't included in Diffusers. Set `trust_remote_code=True` in [`AutoModel.from_pretrained`] to load custom models.
 
+A custom model repository needs a Python module with the model class, and a `config.json` with an `auto_map` entry that maps `"AutoModel"` to `"module_file.ClassName"`.
+
+```
+custom/custom-transformer-model/
+├── config.json
+├── my_model.py
+└── diffusion_pytorch_model.safetensors
+```
+
+The `config.json` includes the `auto_map` field pointing to the custom class.
+
+```json
+{
+  "auto_map": {
+    "AutoModel": "my_model.MyCustomModel"
+  }
+}
+```
+
+Then load it with `trust_remote_code=True`.
+
 ```py
 import torch
 from diffusers import AutoModel
@@ -40,7 +63,39 @@ transformer = AutoModel.from_pretrained(
 )
 ```
 
+For a real-world example, [Overworld/Waypoint-1-Small](https://huggingface.co/Overworld/Waypoint-1-Small/tree/main/transformer) hosts a custom `WorldModel` class across several modules in its `transformer` subfolder.
+
+```
+transformer/
+├── config.json          # auto_map: "model.WorldModel"
+├── model.py
+├── attn.py
+├── nn.py
+├── cache.py
+├── quantize.py
+├── __init__.py
+└── diffusion_pytorch_model.safetensors
+```
+
+```py
+import torch
+from diffusers import AutoModel
+
+transformer = AutoModel.from_pretrained(
+    "Overworld/Waypoint-1-Small", subfolder="transformer", trust_remote_code=True, torch_dtype=torch.bfloat16, device_map="cuda"
+)
+```
+
 If the custom model inherits from the [`ModelMixin`] class, it gets access to the same features as Diffusers model classes, like [regional compilation](../optimization/fp16#regional-compilation) and [group offloading](../optimization/memory#group-offloading).
 
+> [!WARNING]
+> As a precaution with `trust_remote_code=True`, pass a commit hash to the `revision` argument in [`AutoModel.from_pretrained`] to make sure the code hasn't been updated with new malicious code (unless you fully trust the model owners).
+>
+> ```py
+> transformer = AutoModel.from_pretrained(
+>     "Overworld/Waypoint-1-Small", subfolder="transformer", trust_remote_code=True, revision="a3d8cb2"
+> )
+> ```
+
 > [!NOTE]
 > Learn more about implementing custom models in the [Community components](../using-diffusers/custom_pipeline_overview#community-components) guide.