Broadcast mimic for Univariate nodes

[kirill-kondrashov]

In order to properly stack i.i.d. variables in a vector, I suggest to introduce a sort of broadcasting for (some of?) ::Univariate nodes.

Let’s say, I have a distribution, for example, Gamma, which is available only as ::Univariate. I can do that:

D = 2
x = Vector{Variable}(undef, D)
for d in 1:D
  @RV x[d] ~ Gamma(1, 1)
end

But further in the code x is ::Vector{Variable}, so you cannot use it in the model with some kind of vector arithmetics. I'd rather want it as follows:

@RV x ~ Gamma(ones(2), ones(2))

so that further x is a proper vector of two i.i.d. Gamma components.

The broadcast mimic option for Gamma node in this sense follows the spirit of other Julia packages (for instance, I can type pdf.(Gamma(1,1), x) where x can be either Array or scalar).