Implement a working VJP function

ext/LuxReactantExt.jl

@@ -67,28 +67,38 @@ end
 function Lux.__to_reactant_adaptor(
         to::Lux.ToReactantAdaptor{FST}, model::AbstractExplicitLayer,
         input_prototype, ps, st, eltype_adaptor) where {FST}
+    output = first(model(input_prototype, ps, st))
+    concrete_output = __make_concrete_array(output)
+
     concrete_input = __make_concrete_array(input_prototype)
     cmodel = __make_concrete_array(model)
     cps = __make_concrete_array(ps)
     cst = __make_concrete_array(st)
 
     csmodel = Lux.StatefulLuxLayer{FST}(cmodel, cps, cst)
 
-    fwd = Reactant.compile((m, x) -> m(x), (csmodel, concrete_input))
-
-    bwd = try
-        enzyme_grad_fn = (m, x) -> begin
-            dx = Enzyme.make_zero(x)
-            dps = Enzyme.make_zero(m.ps)
-            st = ifelse(FST, m.st, m.st_any)
-            Enzyme.autodiff(
-                Enzyme.Reverse, (m, x, ps, st) -> first(LuxCore.apply(m, x, ps, st)),
-                Enzyme.Duplicated, Enzyme.Const(m.model), Enzyme.Duplicated(x, dx),
-                Enzyme.Duplicated(m.ps, dps), Enzyme.Const(st))
-            return (; ps=dps), dx
+    fwd_fn = Reactant.compile((m, x) -> m(x), (csmodel, concrete_input))
+
+    function enzyme_vjp_fn(m, x, y, dy)
+        dx = Enzyme.make_zero(x)
+        dps = Enzyme.make_zero(m.ps)
+        st_m = ifelse(FST, m.st, m.st_any)
+
+        function wrapper_fn!(y, model, x, ps, st)
+            copyto!(y, first(LuxCore.apply(model, x, ps, st)))
+            return nothing
         end
 
-        Reactant.compile(enzyme_grad_fn, (csmodel, concrete_input))
+        Enzyme.autodiff(Enzyme.Reverse, wrapper_fn!, Enzyme.Const, Enzyme.Duplicated(y, dy),
+            Enzyme.Const(m.model), Enzyme.Duplicated(x, dx),
+            Enzyme.Duplicated(m.ps, dps), Enzyme.Const(st_m))
+        return dx, dps
+    end
+
+    vjp_fn = try
+        concrete_output2 = __make_concrete_array(deepcopy(output))
+        Reactant.compile(
+            enzyme_vjp_fn, (csmodel, concrete_input, concrete_output, concrete_output2))
     catch err
         to.force_compile_backward && rethrow(err)
         @error """
@@ -101,11 +111,9 @@ function Lux.__to_reactant_adaptor(
         nothing
     end
 
-    # TODO: Add compiled types to the layer type information. That way we can check
-    #       if the model is being executed with the correct types.
     return Lux.ReactantLayer{FST, Lux.__recursive_eltype(input_prototype)}(
-        to, input_prototype, concrete_input, cps, cst, model, cmodel, fwd,
-        bwd, eltype_adaptor, fmapstructure(Lux.__size, input_prototype))
+        to, input_prototype, concrete_input, cps, cst, model, cmodel, fwd_fn,
+        vjp_fn, eltype_adaptor, fmapstructure(Lux.__size, input_prototype))
 end
 
 # TODO: Currently we are maintaining 2 copies of the parameters, this is not ideal.
@@ -183,7 +191,7 @@ end
 
 Lux.__apply_reactant(l, x, ps, st) = __graceful_type_mismatch_error(l, x, ps, st)
 
-@inline Lux.__apply_reactant(l::Lux.ReactantLayer, csmodel, x) = l.fwd(csmodel, x)
+@inline Lux.__apply_reactant(l::Lux.ReactantLayer, csmodel, x) = l.fwd_fn(csmodel, x)
 
 # Don't inline, else types don't get displayed in the stack trace
 function __graceful_type_mismatch_error(

src/layers/extension.jl

@@ -245,7 +245,8 @@ end
 
 # TODO: Add a ChainRules rrule that calls the `bwd` function, i.e. uses Enzyme for the
 #       gradient computation
-@concrete struct ReactantLayer{FST, T, inType, inCType, psType, stType, F, B,
+# TODO: Inference won't work OOTB, we will have to compile that separately
+@concrete struct ReactantLayer{FST, T, inType, inCType, psType, stType,
     L <: AbstractExplicitLayer, AD <: ToReactantAdaptor} <: AbstractExplicitLayer
     adaptor::AD
     input_prototype::inType
@@ -254,8 +255,8 @@ end
     concrete_st::stType
     layer::L
     clayer
-    fwd::F
-    bwd::B
+    fwd_fn
+    vjp_fn
     eltype_adaptor
     input_structure
 end