By Hyunjoo Jin SAN FRANCISCO (Reuters) -Tesla CEO Elon Musk on Friday showed off a prototype of humanoid robot ‘Optimus’, predicting the electric vehicle maker would be able to produce millions of them and sell them for under $20,000 – less than a third of the price of a Model Y. “There’s still a lot of work to be done to refine Optimus and prove it,” Musk told the electric vehicle maker’s “AI Day” event being held at a Tesla office in Palo Alto, California, where the robot was showcased. A prototype model that Tesla said was developed in February walked out to wave at the crowd on Friday, and…