Add instructions to create DataLoader from augmented dataset in object detection guide

[Laurent2916]

The following adds instructions on how to create a DataLoader from the guide on how to use object detection with augmentations (#4710). I am open to hearing any suggestions for improvement !

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[mariosasko]

I'm not sure we need this part as we provide a link to the notebook that shows how to train an object detection model, and this notebook instantiates a DataLoader before training the model. I'd like to hear what @stevhliu thinks.

PS: Your collate_fn calls torch.stack on the bbox tensors, which don't have the same shape, so this will fail.

[stevhliu]

I agree with @mariosasko; we also have a Use with PyTorch guide that shows how you can create a DataLoader.