add merge blocking to ci: sev template (#83940)
as in title, so that by default, ci: sev will block merges
the line can be removed to not block merges
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83940
Approved by: https://github.com/huydhn, https://github.com/janeyx99, https://github.com/malfet, https://github.com/seemethere
