[checkpoint] Improve test (test_nested_dict.py) (#89854)
Improve the test_nested_dict.py test:
1. Add comments to show flatten_dict and mapping result.
2. Update test_mapping unit test to ensure the key value pair matching in mapping.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89854
Approved by: https://github.com/H-Huang
