Every day, Mercor, a startup that sells training data to artificial intelligence companies, pays 30,000 contractors more than $4 million to help make their jobs, and those of their colleagues, obsolete. It’s gig work, but for professionals with rarefied skills. One recent Mercor posting offered $225 an hour for a voice actor able to maintain a customer service persona in fluent Hebrew. Another sought a Ph.D. physicist with a specialization in general relativity, astrophysics or cosmology.