Make CPUThreadPoolExecutor::add(_, _, _, _) virtual [RFC]
Make CPUThreadPoolExecutor::add(_, _, _, _) virtual [RFC]

Summary: I want to override this method in the next [diff](https://www.internalfb.com/diff/D31995497?entry_point=27), so I am making it virtual

Reviewed By: yfeldblum, jackerghan

Differential Revision: D31995385

fbshipit-source-id: 20855cfee0bcf809bdd477ff4820613180986178

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