Let connection in Parallel be a layer

[vpuri3]

This seems to be a common use of Parallel/ BranchLayer containers:

Chain(
  BranchLayer(chain1, chain2),       # x -> (f(x), g(x))
  Parallel(nothing, chain3, chain4), # (x1, x2) -> (f(x1), g(x2))
  fusion                             # (y1, y2) -> z eg Bilinear layer
)

an easier syntax for the last two layers would be Parallel(fusion, chain1, chain2) which entails allowing connection in Parallel to be a Lux layer. Likely adding an if C <: AbstractExplicitLayer statement here.

Lux.jl/src/layers/containers.jl

Lines 162 to 166 in 2dde6f8

	if C == Nothing
	push!(calls, :($(y_symbols[N + 1]) = tuple($(Tuple(y_symbols[1:N])...))))
	else
	push!(calls, :($(y_symbols[N + 1]) = connection($(Tuple(y_symbols[1:N])...))))
	end

Similarly, BranchLayer can be extended with kwarg fusion like BranchLayer(layers...; fusion = nothing) with behaviour x -> fusion(f(x), g(x)). This is equivalent to

Chain(
  BranchLayer(chain1, chain2), # x -> (f(x), g(x))
  fusion,                 # (x1, x2) -> z
)

The default can be set to the current behaviour (x -> (f(x), g(x))) with fusion = nothing.