Jerry Yang is leaving Yahoo, the Internet company he helped start 17 years ago. He did not cite a reason. In a statement, Roy Bostock, Yahoo’s chairman, said Mr. Yang would immediately give up his board seat at Yahoo and step down from the boards of the Alibaba Group and Yahoo Japan.Mr. Yang did not give a reason for his departure, but it occurred as the company undergoes a strategic review under a new chief executive, Scott Thompson, on whether the company should sell off its Asian interests and focus on its media assets. Yahoo owns a 40 percent stake in Alibaba and a 35 percent stake in Yahoo Japan.