Mark istrans as inactive (#658)

* Mark istrans as inactive

* Update DynamicPPLEnzymeCoreExt.jl

* Update DynamicPPLEnzymeCoreExt.jl

* Expand Enzyme extension comment

---------

Co-authored-by: Markus Hauru <mhauru@turing.ac.uk>

Author: wsmoses

ext/DynamicPPLEnzymeCoreExt.jl

@@ -10,4 +10,9 @@ end
 
 @inline EnzymeCore.EnzymeRules.inactive_type(::Type{<:DynamicPPL.SamplingContext}) = true
 
+# Mark istrans as having 0 derivative. The `nothing` return value is not significant, Enzyme
+# only checks whether such a method exists, and never runs it.
+@inline EnzymeCore.EnzymeRules.inactive_noinl(::typeof(DynamicPPL.istrans), args...) =
+    nothing
+
 end