.circleci: Only do comparisons when available (#42816)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42816
Comparisons were being done on branches where the '<<
pipeline.git.base_revision >>' didn't exist before so let's just move it
so that comparison / code branch is only run when that variable is
available
Example: https://app.circleci.com/pipelines/github/pytorch/pytorch/198611/workflows/8a316eef-d864-4bb0-863f-1454696b1e8a/jobs/6610393
Signed-off-by: Eli Uriegas <eliuriegas@fb.com>
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D23032900
Pulled By: seemethere
fbshipit-source-id: 98a49c78b174d6fde9c6b5bd3d86a6058d0658bd
