message: Fix generated documentation for arguments that default to "unknown shape".
diff: @@ -326,6 +326,9 @@ string DataTypeToPython(DataType dtype, const string& dtype_module) {
  }
  string ShapeToPython(const TensorShapeProto& shape) {
  + if (shape.unknown_rank()) {
  + return "None";
  + }
  string python = "[";
  for (const auto& dim : shape.dim()) {
  if (python.size() > 1) strings::StrAppend(&python, ", ");
  