gen_backend_stubs.py: fix typo for supported_autograd (#68562)
Summary:
Fixes #{issue number}
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68562
Reviewed By: jbschlosser
Differential Revision: D32758608
Pulled By: bdhirsh
fbshipit-source-id: 496e1ec831edaa6fcc586f3c8f0361c31cad4e78
(cherry picked from commit 68ea9e9df568d23f31e8466d6e01c046a6c1ff0f)
