message: Add note regarding fixed types in NodeTypeAttrMap
diff: @@ -287,6 +287,10 @@ class NodeTypeAttrMap {
  input_arg_inds.size(), " non-control input(s), but got ",
  node.input_size());
  }
  + // Note that the mappings generated here include inputs/outputs with fixed
  + // types. This makes the mappings complete (all inputs and outputs are
  + // included), and allows the graph rewriter to propagate black paint
  + // from/through ops with fixed types.
  io2type_entry.first.reserve(input_arg_inds.size());
  for (int i = 0; i < (int)input_arg_inds.size(); ++i) {
  const auto& arg_inds = input_arg_inds[i];
  