Fix missing bijector import

Author: penelopeysm

test/variational/advi.jl

@@ -4,6 +4,7 @@ using ..Models: gdemo_default
 using ..NumericalTests: check_gdemo
 import AdvancedVI
 using AdvancedVI: TruncatedADAGrad, DecayedADAGrad
+using Bijectors: bijector
 using Distributions: Dirichlet, Normal
 using LinearAlgebra: I
 using MCMCChains: Chains