Frozen Weights

[avik-pal]

FluxML/Optimisers.jl#112 introduces support for frozen weights natively in Optimisers.jl. We still want to retain our current experimental API and just show how to use this native feature in a tutorial (needs overloading the @layer_map macro).

[avik-pal]

The experimental API has been around for quite sometime now, and there hasn't been any issues. I will still keep it marked as experimental but we can consider this as solved.