Fix missing load_video documentation and load_video import in WanVideoToVideoPipeline example code (#12472)

* Update utilities.md

Update missing load_video documentation

* Update pipeline_wan_video2video.py

Fix missing load_video import in example code

docs/source/en/api/utilities.md

@@ -26,6 +26,10 @@ Utility and helper functions for working with 🤗 Diffusers.
 
 [[autodoc]] utils.load_image
 
+## load_video
+
+[[autodoc]] utils.load_video
+
 ## export_to_gif
 
 [[autodoc]] utils.export_to_gif