heterogeneity (#27)

Prepare things to work with heterogeneity, make things work on GPU

Reviewed-on: Rubydragon/MetagraphOptimization.jl#27
Co-authored-by: Anton Reinhard <anton.reinhard@proton.me>
Co-committed-by: Anton Reinhard <anton.reinhard@proton.me>

src/optimization/reduce.jl

@@ -28,3 +28,7 @@ function optimize_to_fixpoint!(optimizer::ReductionOptimizer, graph::DAG)
     end
     return nothing
 end
+
+function String(::ReductionOptimizer)
+    return "reduction_optimizer"
+end