Updates to computer vision section of the Preprocess doc
This PR expands the Computer Vision section of the Preprocess doc to include a small explainer on the difference between image augmentation and image preprocessing, and what ImageProcessor handles. It also refactors the code example to use `ImageProcessor` for normalizing and converting images to tensors instead of `torch.transforms`. 
 It mentions padding for certain cases (DETR), and the availability of post-processing methods for some models/tasks. 

_The documentation is not available anymore as the PR was closed or merged._