Migrate `federated_research/large_cohort` to the `tff.programs` managers.
Migrate `federated_research/large_cohort` to the `tff.programs` managers.

PiperOrigin-RevId: 416868276
Change-Id: I883de36a4fff5cfc9bee4f9b622265e7f257d2be

google-research/federated,google-research/federated