Add CheckpointManager to keep avg checkpoint weights in memory to reduce disk...

Add CheckpointManager to keep avg checkpoint weights in memory to reduce disk read when averaging + various checkpoint refactoring

Summary: Pull Request resolved: https://github.com/pytorch/translate/pull/315

Reviewed By: akinh

Differential Revision: D13510446

fbshipit-source-id: 22a6594af9253130a93e638285a47183a974e0de