Accept multiple differentiable inputs in maximizers and multiple outputs

[BatyLeo]

Currently, maximizers are only allowed one \theta input. It could be interesting and practical for some applications to support multiple input of various sizes. In this case, multiple outputs are needed to be able to reconstruct the objective value as a sum of pairwise dot products.

Example:

function my_maximizer(theta, eta; kwargs...)
    ...
   return y1, y2
end

with objective being dot(theta, y1) + dot(eta, y2)

In some cases, this would avoid annoying concatenation and flattening pre/post processing.