ReverseDiff Extension makes vector parameter into a scalar

[avik-pal]

If I provide a parameter p = [4.0], I would expect it to stay as a Vector. Currently it gets converted into a TrackedReal.

julia> reduce(vcat, ReverseDiff.track([4.0]))
TrackedReal<Cz3>(4.0, 0.0, 5TV, 1, 3KQ)

Maybe we should add a special case for length(p) == 1 and not do reduce(vcat, p)

[ChrisRackauckas]

julia> using ReverseDiff

julia> typeof(reduce(vcat, ReverseDiff.track([4.0])))
ReverseDiff.TrackedReal{Float64, Float64, ReverseDiff.TrackedArray{Float64, Float64, 1, Vector{Float64}, Vector{Float64}}}

julia> typeof(reduce(vcat, ReverseDiff.track([4.0,4.0])))
ReverseDiff.TrackedArray{Float64, Float64, 1, Vector{Float64}, Vector{Float64}}

I'd surface that as a bug in ReverseDiff.

[avik-pal]

julia> reduce(vcat, [4.0])
4.0

That holds even for regular arrays