[booster] add tests for ddp and low level zero's checkpointio

[flybird11111]

📌 Checklist before creating the PR

• I have created an issue for this PR for traceability
• The title follows the standard format: [doc/gemini/tensor/...]: A concise description
• I have added relevant tags if possible for us to better distinguish different PRs

🚨 Issue number

Link this PR to your issue with words like fixed to automatically close the linked issue upon merge

e.g. fixed #1234, closed #1234, resolved #1234

resolved #3716

📝 What does this PR do?

Summarize your work here.
if you have any plots/diagrams/screenshots/tables, please attach them here.

this work is to improve booster unit tests. I add some tests of torchDDPPlugin's and LowLevelZeroPlugin's CheckpointIO.

💥 Checklist before requesting a review

• I have linked my PR to an issue (instruction)
• My issue clearly describes the problem/feature/proposal, with diagrams/charts/table/code if possible
• I have performed a self-review of my code
• I have added thorough tests.
• I have added docstrings for all the functions/methods I implemented

⭐️ Do you enjoy contributing to Colossal-AI?

• 🌝 Yes, I do.
• 🌚 No, I don't.

Tell us more if you don't enjoy contributing to Colossal-AI.

[github-actions]

The code coverage for the changed files is 98%.

Click me to view the complete report

Name                                                            Stmts   Miss  Cover
-----------------------------------------------------------------------------------
colossalai/testing/__init__.py                                      4      0   100%
colossalai/testing/comparison.py                                   38      6    84%
tests/test_checkpoint_io/test_gemini_checkpoint_io.py              72      0   100%
tests/test_checkpoint_io/test_general_checkpoint_io.py             57      0   100%
tests/test_checkpoint_io/test_low_level_zero_checkpoint_io.py      40      0   100%
tests/test_checkpoint_io/test_torch_ddp_checkpoint_io.py           49      0   100%
-----------------------------------------------------------------------------------
TOTAL                                                             260      6    98%