message: Add un-init placeholder warning to negative-dimension error
diff: @@ -359,7 +359,8 @@ Status TensorShapeOld::IsValidShape(const TensorShapeProto& proto) {
  for (const auto& d : proto.dim()) {
  if (d.size() < 0) {
  return errors::InvalidArgument("Shape ", DebugString(proto),
  - " has negative dimensions");
  + " has negative dimensions; ",
  + "perhaps an uninitialized placeholder?");
  }
  num_elements *= d.size();
  if (num_elements > kMaxElements) {
  