Add IS_PYTORCH_CI flag for testing (#16006)
Summary:
Use case:
Some data loader tests rely on `psutil` (a third party lib). So they are guarded by `skipIf`. But we want to always test them on CI envs. With `IS_PYTORCH_CI`, we can raise if `psutil` is not found.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16006
Reviewed By: ezyang
Differential Revision: D13673957
Pulled By: yf225
fbshipit-source-id: c63a7138093f45333c0b371fed0bcc88b67f2a22
