Moving original grpo_actor with new changes into docs so we can keep edit history. (#922)

Summary:
Pull Request resolved: #922

Move the grpo_actor.py with change history back into the docs location (so it matches what it was, but we recovered change history in the prior few commits, which had been lost).

Reviewed By: shayne-fletcher

Differential Revision: D80534399

fbshipit-source-id: 730c97578f9b6f2586a33e5b5d9a1df27082a4a9

Author: johnwhumphreys