Consider adding FrontendAttributesAttr and BackendConfigAttr #628

  if (proto.has_frontend_attributes()) {
    instruction->set_frontend_attributes(proto.frontend_attributes());
  }

  // Generic map of attributes used to pass hints / configuration options from
  // the Python frontend to the XLA backend.
  message FrontendAttributes {
    map<string, string> map = 1;
  }

  instruction->backend_config_ = proto.backend_config();

  // Backend configuration for the instruction. Has backend-specific meaning.
  bytes backend_config = 43;