Use Metalhead.jl for the vision examples

[theabhirath]

Currently, the examples for ConvMixer and VGG have the code for the models written inside them. Since Metalhead has cleaner versions of these models, would it make sense to replace these or are these code examples kept specifically?

[DhairyaLGandhi]

We should keep the VGG example to show how to write out many of the traditional models, but use Metalhead for using predefined models separately. Great idea!