Checkpoint part 1: create a separate buck target

Summary:
Pull Request resolved: https://github.com/facebookresearch/d2go/pull/556

The TARGETS for the files inside the directory `checkpoint` are tackled in two parts:

1. This diff creates TARGETS for the files inside `checkpoint` i.e. except `checkpoint/fb/tests`
2. The diff D46096372 creates TARGETS for files inside `checkpoint/fb/tests`

Reviewed By: tglik, wat3rBro

Differential Revision: D45912080

fbshipit-source-id: 04ab44e015a9d89d18e31c854600df05d35539d1

d2go/runner/default_runner.py

@@ -11,7 +11,8 @@ from typing import List, Optional, Type, Union
 import d2go.utils.abnormal_checker as abnormal_checker
 import detectron2.utils.comm as comm
 import torch
-from d2go.checkpoint import FSDPCheckpointer, is_distributed_checkpoint
+from d2go.checkpoint.api import is_distributed_checkpoint
+from d2go.checkpoint.fsdp_checkpoint import FSDPCheckpointer
 from d2go.config import CfgNode, CONFIG_SCALING_METHOD_REGISTRY, temp_defrost
 from d2go.config.utils import get_cfg_diff_table
 from d2go.data.build import build_d2go_train_loader