message: show nodes with gradients present
diff: @@ -1331,6 +1331,8 @@ template <typename Value> const char *ad_graphviz() {
  color = "lightblue2";
  if (labeled && !color)
  color = "wheat";
  + if (is_valid(v->grad))
  + color = "yellowgreen";
  buffer.fmt("|{a%i|S:%u|E:%u|I:%u%s}",
  index, v->size, v->ref_count_ext,
  @@ -1373,6 +1375,7 @@ template <typename Value> const char *ad_graphviz() {
  buffer.put(
  " subgraph cluster_legend {\n"
  " label=\"Legend\";\n"
  + " l4 [style=filled fillcolor=yellowgreen label=\"Gradient present\"];\n"
  " l3 [style=filled fillcolor=salmon label=\"Input\"];\n"
  " l2 [style=filled fillcolor=lightblue2 label=\"Output\"];\n"
  " l1 [style=filled fillcolor=wheat label=\"Labeled\"];\n"
  