Having the knowledge of God in His brain, what earthly training could possibly suit Him? Are you suggesting that He did not sound learned and that His understanding was not up to that of a Rabbi?
John’s Gospel indicates that somehow the Jewish authorities knew that Jesus had not received formal rabbinical training, and they were using that against him to challenge his authority to teach. It would not have been unusual for his followers to call him Rabbi as a courtesy and in a non-technical sense, despite his lack of such formal training. (I agree that he hardly needed it.)