minor improvement: extract major version (#49393)
Summary:
1. Unify major version extraction. If there's an error, it would throw exception in PR CI. So far, not all cuda version tests are in PR CI
2. better readability.
passed cuda11 tests.
https://circleci.com/gh/pytorch/pytorch/9651144?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link
https://circleci.com/gh/pytorch/pytorch/9651145?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49393
Reviewed By: zou3519
Differential Revision: D25828318
Pulled By: seemethere
fbshipit-source-id: 5c6861f0ddafe9a77a9fe397e4e0f69ecce4b27f
