Fix type annotation error. (#15448)
Summary:
According to mypy, the trailing -> None is mandatory.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15448
Differential Revision: D13532179
Pulled By: ezyang
fbshipit-source-id: e8972f8c9ada4657c518cd7bcd46e489ab8ddf5f
