Turn namedtensor build back on (#21520)
Summary:
namedtensor build + test should run on PRs only if the commit message
includes [namedtensor ci].
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21520
Differential Revision: D15718404
Pulled By: zou3519
fbshipit-source-id: ce8b5df2682e795e64958a9d49e2e3c091599b33
