Fix "Missing return statement" mypy error (#49276)
Summary:
Adds `return None` after `assert_never` in the inner `get_one` function
Without it, TestTypeHints.test_run_mypy_strict using mypy 0.770 fails with the above mentioned error, see https://app.circleci.com/pipelines/github/pytorch/pytorch/249909/workflows/597d8e34-ff04-4efa-9dde-9e28fbded341/jobs/9557705
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49276
Reviewed By: jamesr66a
Differential Revision: D25513658
Pulled By: malfet
fbshipit-source-id: 318eaff7e0534b10eafe46c0b834b7f7cefea757
