fix: support Colon() in middle of indices for NodeLabel (#275)

[ArpanC6]

Fixes #275
Fixes #293

While working with multi-dimensional latent variables in a model I noticed that using a colon : in the middle of an index expression like media[g, :, i] causes a crash with a MethodError. For example, this model

@model function my_model(y)
    media ~ Normal(zeros(2, 5, 3), ones(2, 5, 3))
    for g in 1:2
        for i in 1:3
            media_effect[g, i] := media[g, :, i]
        end
    end
    y ~ Normal(0, 1)
end

would throw

MethodError: no method matching getindex(::GraphPPL.NodeLabel, ::Int64, ::Colon, ::Int64)

The issue is that NodeLabel only had a single-argument getindex overload:

Base.getindex(label::NodeLabel, any) = label

When Julia sees media[g, :, i] on a NodeLabel, it tries to call getindex with three arguments, which didn't have a matching method. Since NodeLabel represents a node in the factor graph and not an actual array the right behaviour is to just return the label itself regardless of the indices which is exactly what the existing single index overload already does.

The fix is a one-line addition:

Base.getindex(label::NodeLabel, i1, is...) = label

I also added a test that covers this case to make sure it doesn't regress.