add base checkpointer class (#626)

Summary:
Pull Request resolved: #626

# Context
We want to add another checkpointer using [DCP](https://pytorch.org/docs/stable/distributed.checkpoint.html). However, we don't want to duplicate the logic that already exists in TorchSnapshotSaver related to checkpoint frequency, keeping k latest checkpoints, etc

# This Diff
* Adds abstract `BaseCheckpointer` class to implements common logic like syncing dirpath's across all ranks, implementing all hooks where checkpoint may occur, etc.

* Any class subclassing must implement `_checkpoint_impl` and `restore` functions. The `restore_from_latest` method will call the user defined `restore`.

* copies all applicable tests from `TorchSnapshotSaver` into `BaseCheckpointer`'s test (will remove relevant `TorchSnapshotSaver` tests in next diff)

Reviewed By: galrotem

Differential Revision: D51328340

fbshipit-source-id: b7bc65c294fabf5d3671735dc1afcf54c1c59a1b