Support NamedTuples for Container Layers

[ToucheSir]

This came up in FluxML/Metalhead.jl#70 and surprised me, because I thought we supported this already!

Other than Chain, are there any other layers we'd want to support this? The only other one that uses a tuple field is Parallel, but since it doesn't forward methods to .layers I'm not sure this is applicable.

Another thing using NamedTuples would open up is the possibility of defining aliases for Chain. e.g.

const ResNetBlock{C1,C2,N} = Chain{NamedTuple{(:conv1, :conv2, :norm), Tuple{C1, C2, N}}}

[darsnack]

I think this in combo with @mcabbott's "big show" would make models really readable. Like big show can print the names alongside the other info.

Is forwarding methods conceptually linked here (am I missing something)? For Parallel it would be nice to name the branches. I could see this being really useful for inception modules.

[DhairyaLGandhi]

That works already in the sense that we can optionally store named tuples, what was spoken about in the discussion was how to leverage it for larger models.

Forwarding indexing would be needed. The printing is really to have custom show methods, since the Chain aren't as meaningful as the concept they represent.

[darsnack]

Chain is one of those weird layers in that its types/constructor are extremely restrictive:

Flux.jl/src/layers/basic.jl

Lines 26 to 29 in 0a21546

	struct Chain{T<:Tuple}
	layers::T
	Chain(xs...) = new{typeof(xs)}(xs)
	end

[darsnack]

I think Parallel does forward indexing to the branches?