Add stateless_apply

This calls `apply` and only returns the first argument.

src/LuxCore.jl

@@ -126,6 +126,21 @@ Simply calls `model(x, ps, st)`
 """
 apply(model::AbstractExplicitLayer, x, ps, st) = model(x, ps, st)
 
+"""
+    stateless_apply(model, x, ps, st)
+
+Calls `apply` and only returns the first argument.
+"""
+function stateless_apply(model::AbstractExplicitLayer, x, ps, st)
+    u, st = apply(model, x, ps, st)
+    @assert isempty(st) "Model is not stateless. Use `apply` instead."
+    return u
+end
+
+function stateless_apply(::StatefulLuxLayer, x, ps, st)
+    return error("Model is not stateless. Use `apply` instead.")
+end
+
 """
     display_name(layer::AbstractExplicitLayer)