Modernize to default constructor and nullptr in torch (#61735)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/61735
Test Plan: Sandcastle
Reviewed By: malfet
Differential Revision: D29716659
fbshipit-source-id: ec2a0a0b7e55d2e50b1d35f0b651bd40675ae7e8
