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 transfered the copyright to a third party such as the company or school you work for.
Authors do not tranfer the copyright of their paper 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