Support dictionary of parameters

[freddycct]

Describe the potential feature

Optimisers.setup(Optimisers.Adam(), Dict(1 => rand(Float32, 5), 2=>rand(Float32, 5))) currently returns empty tuple ()

Motivation

Would help more complicated neural architectures.

Possible Implementation

No response

[ToucheSir]

We'd have to decide how/if to support Dicts in Functors first. Can you open an issue to this effect there?

[mcabbott]

Can you explain more about why or where Dict is a good idea, instead of say a NamedTuple? Is its mutability important? Are the keys things you invent, or is this something more like an IdDict?

[CarloLucibello]

I have a use case in
CarloLucibello/GraphNeuralNetworks.jl#146
where the keys are tuples of string, using named tuple instead would be awkward.
There could be some exotic situations where the dict's values are trained, although most of the time they contain static features. In any case though they have to be moved to gpus, so functor support is necessary.