code: def send_to_device(self, event_type, messages, txn_id=None):
          
          txn_id = txn_id if txn_id else self._make_txn_id()
          return self._send(
              "PUT",
              "/sendToDevice/{}/{}".format(event_type, txn_id),
              content={"messages": messages}
          )
docs: Sends send-to-device events to a set of client devices.
  
          Args:
              event_type (str): The type of event to send.
              messages (dict): The messages to send. Format should be
                  <user_id>: {<device_id>: <event_content>}.
                  The device ID may also be '*', meaning all known devices for the user.
              txn_id (str): Optional. The transaction ID for this event, will be generated
                  automatically otherwise.