Yu Ruidong/China News Service/Visual China Group via Getty Images AI's boost to productivity could be transformational or just marginal, Bank of America said. If it's transformational, BofA said to buy the providers of the technology. But if it's more marginal, then buy companies that are effectively implementing AI. The productivity boost from artificial intelligence could be transformational or marginal, and investors should pick stocks accordingly, Bank of America said. If it's transformational, BofA said to buy the providers of the technology. But since it's hard to pick the eventual winners, strategists pointed to AI-themed ETFs. The positive impact of AI…