Amazon's self-driving unit Zoox started offering free rides to some early users in parts of San Francisco, it said on Tuesday, as it steps up expansion amid rising competition in the autonomous ride-hailing sector. Tesla's launch of its robotaxi service this year and accelerated expansion by Alphabet's Waymo have renewed focus on commercializing autonomous vehicles, even as high investments, tight regulations and federal investigations have forced many to shut down. Zoox is inviting people from