Replacing obsolete function in documentation

[oussamachaib]

The Datasets & DataLoaders tutorial uses an obsolete function (read_image) to decode digital images and convert them into torch tensors. I suggest replacing the function by its up-to-date equivalent (decode_image).

I have made a pull request.

cc @datumbox @nairbv @fmassa @NicolasHug @YosuaMichael

[svekars]

Thank you @oussamachaib! Can you please submit against the tutorial repo.