Mark function as Const for Enzyme tests (#957)

Author: penelopeysm

test/integration/enzyme/main.jl

@@ -2,12 +2,14 @@ using DynamicPPL.TestUtils: DEMO_MODELS
 using DynamicPPL.TestUtils.AD: run_ad
 using ADTypes: AutoEnzyme
 using Test: @test, @testset
-import Enzyme: set_runtime_activity, Forward, Reverse
+import Enzyme: set_runtime_activity, Forward, Reverse, Const
 using ForwardDiff: ForwardDiff  # run_ad uses FD for correctness test
 
 ADTYPES = Dict(
-    "EnzymeForward" => AutoEnzyme(; mode=set_runtime_activity(Forward)),
-    "EnzymeReverse" => AutoEnzyme(; mode=set_runtime_activity(Reverse)),
+    "EnzymeForward" =>
+        AutoEnzyme(; mode=set_runtime_activity(Forward), function_annotation=Const),
+    "EnzymeReverse" =>
+        AutoEnzyme(; mode=set_runtime_activity(Reverse), function_annotation=Const),
 )
 
 @testset "$ad_key" for (ad_key, ad_type) in ADTYPES