[`Doc`] Fix docbuilder - make `BackboneMixin` and `BackboneConfigMixin`...

[`Doc`] Fix docbuilder - make `BackboneMixin` and `BackboneConfigMixin` importable from `utils`.  (#29002)

* Trigger doc build

* Test removing references

* Importable from utils

* Trigger another run on a new commit for testing

docs/source/en/main_classes/backbones.md

@@ -18,8 +18,8 @@ rendered properly in your Markdown viewer.
 
 A backbone is a model used for feature extraction for higher level computer vision tasks such as object detection and image classification. Transformers provides an [`AutoBackbone`] class for initializing a Transformers backbone from pretrained model weights, and two utility classes:
 
-* [`~utils.backbone_utils.BackboneMixin`] enables initializing a backbone from Transformers or [timm](https://hf.co/docs/timm/index) and includes functions for returning the output features and indices.
-* [`~utils.backbone_utils.BackboneConfigMixin`] sets the output features and indices of the backbone configuration.
+* [`~utils.BackboneMixin`] enables initializing a backbone from Transformers or [timm](https://hf.co/docs/timm/index) and includes functions for returning the output features and indices.
+* [`~utils.BackboneConfigMixin`] sets the output features and indices of the backbone configuration.
 
 [timm](https://hf.co/docs/timm/index) models are loaded with the [`TimmBackbone`] and [`TimmBackboneConfig`] classes.
 
@@ -45,11 +45,11 @@ Backbones are supported for the following models:
 
 ## BackboneMixin
 
-[[autodoc]] utils.backbone_utils.BackboneMixin
+[[autodoc]] utils.BackboneMixin
 
 ## BackboneConfigMixin
 
-[[autodoc]] utils.backbone_utils.BackboneConfigMixin
+[[autodoc]] utils.BackboneConfigMixin
 
 ## TimmBackbone
 

src/transformers/utils/__init__.py

@@ -20,6 +20,7 @@ from huggingface_hub.constants import HF_HUB_DISABLE_TELEMETRY as DISABLE_TELEME
 from packaging import version
 
 from .. import __version__
+from .backbone_utils import BackboneConfigMixin, BackboneMixin
 from .constants import IMAGENET_DEFAULT_MEAN, IMAGENET_DEFAULT_STD, IMAGENET_STANDARD_MEAN, IMAGENET_STANDARD_STD
 from .doc import (
     add_code_sample_docstrings,