gather on a sparse source should return a dense array

[CarloLucibello]

This is the current behavior:

julia> s = [1, 2, 3];

julia> t = [2, 3, 1];

julia> A = sparse(s, t, [1.0, 1.0, 1.0], 3, 3)
3×3 SparseMatrixCSC{Float64, Int64} with 3 stored entries:
  ⋅   1.0   ⋅ 
  ⋅    ⋅   1.0
 1.0   ⋅    ⋅ 

julia> w = NNlib.gather(A, s, t)
3-element SparseVector{Float64, Int64} with 3 stored entries:
  [1]  =  1.0
  [2]  =  1.0
  [3]  =  1.0

I would expect w to be a dense vector instead.