[caffe2] add a basic implementation of run-time feature rollout checks (#59355)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59355
Add a `CheckKnob()` function for doing run-time checks of feature roll-out
knobs. This provides an API for safely controlling the roll-out of new
functionality in the code.
Test Plan: Included some basic unit tests.
Reviewed By: voznesenskym
Differential Revision: D26536430
fbshipit-source-id: 2e53234c6d9ce624848fc8b2c76f6833f344f48b
