BOSTON (AP) — Amazon unveiled plans on Tuesday for a major expansion in Boston's Seaport District, promising 2,000 new technology jobs even as the city remains in contention for the company's coveted second headquarters. Seattle-based Amazon announced it would move into a 430,000-square-foot "Tech Hub" in 2021, and said the new jobs would be created in fields including machine learning, speech science, cloud computing, and robotics. The company currently has about 1,200 workers in the city, according to Rohit Prasad, a Boston-based vice president and head scientist of Amazon Alexa.