rename reference state dicts (#838)

## Description

Use clearer variable names for reference state dicts in the checkpoint
test.

## Checklist:

- [x] Have you added tests that prove your fix is effective or that this
feature works?
- [x] Has code been commented, particularly in hard-to-understand areas?

Co-authored-by: Eddy <ogolaedd@meta.com>

test/local_test_checkpoint.py

@@ -96,8 +96,8 @@ def run_worker(args: List[str | int]) -> None:
 
     # Take an optimization step
     optimizer.step()
-    submod_ref = deepcopy(stage.submod.state_dict())
-    optim_ref = deepcopy(optimizer.state_dict())
+    ref_state_dict = deepcopy(stage.submod.state_dict())
+    ref_optim_state_dict = deepcopy(optimizer.state_dict())
     save_checkpoint(stage, CKPT_DIR, optimizer)
 
     # save index file in rank 0