Apple Inc.’s Robby Walker, one of the iPhone maker’s most senior artificial intelligence executives, is leaving the company, according to people with knowledge of the matter. Walker, who’s one of a few direct reports to AI chief John Giannandrea, was in charge of Siri until earlier this year — before oversight of the voice assistant was shifted to software chief Craig Federighi. That move came after promised changes to the feature were publicly postponed, marking a high-profile setback....