Outdated DataLoader tutorial

[CarloLucibello]

The [Dataloader documentation] points to a tutorial page, https://fluxml.ai/tutorials/2021/01/21/data-loader.html, which is a bit outdated. In particular, the old MLDatasets.jl interface is used

x, y = MNIST.traindata(Float32)
dataloader = DataLoader((x, y))

instead of the new one

x, y = MNIST()[:]
dataloader = DataLoader((x, y))

[Saransh-cpp]

@CarloLucibello could you move this issue to https://github.com/FluxML/fluxml.github.io? The tutorial originates from there and https://github.com/FluxML/fluxml.github.io/pull/146 should close this!

[CarloLucibello]

Closed in #146