Make Perf estimator easy to be inherited (#1718)

Summary:
Pull Request resolved: #1718

Only doing it for Perf estimator for now.

The idea is we want users to be able to substitute their estimators.

Reviewed By: damianr99

Differential Revision: D53674046

fbshipit-source-id: 849bf4212879ab085aac86ade14e0e33d1d35783