Delay external imports until we're ready to test tensorboard (#25993)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25993
These imports fail the test suite if they're not installed, even if we
don't end up testing tensorboard.
[test macos]
Test Plan: Imported from OSS
Differential Revision: D17318588
Pulled By: pietern
fbshipit-source-id: febad497ecb3fd292317f68fc2439acd893ccd67
