Who holds the Copyright on a NeurIPS paper

According to U.S. Copyright Office's page, What is a Copyright, when you create an original work you are the author and the owner and hold the copyright, unless you have an agreement to transfer the copyright to a third party such as the company or school you work for.

Authors do not transfer the copyright of their papers to NeurIPS. Instead, they grant NeurIPS a non-exclusive, perpetual, royalty-free, fully-paid, fully-assignable license to copy, distribute and publicly display all or part of the paper.