Start unittesting our main observer (#15191)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15191
OSS:
just splitting out basic flags from a unit test. So I can extend them in another test where I need to add additional flags.
Reviewed By: yinghai
Differential Revision: D13159184
fbshipit-source-id: 9823e792cf0ed8d0379235c44564862b7d784845
