Add QED Model (#25)

Reviewed-on: Rubydragon/MetagraphOptimization.jl#25
Co-authored-by: Anton Reinhard <anton.reinhard@proton.me>
Co-committed-by: Anton Reinhard <anton.reinhard@proton.me>

src/task/properties.jl

@@ -58,6 +58,29 @@ Return the data of a data task. Given by the task's `.data` field.
 """
 data(t::AbstractDataTask)::Float64 = getfield(t, :data)
 
+"""
+    copy(t::DataTask)
+
+Copy the data task and return it.
+"""
+copy(t::DataTask) = DataTask(t.data)
+
+"""
+    children(::DataTask)
+
+Return the number of children of a data task (always 1).
+"""
+children(::DataTask) = 1
+
+"""
+    children(t::FusedComputeTask)
+
+Return the number of children of a FusedComputeTask.
+"""
+function children(t::FusedComputeTask)
+    return length(union(Set(t.t1_inputs), Set(t.t2_inputs)))
+end
+
 """
     data(t::AbstractComputeTask)