[checkpoint] refactored the API and added safetensors support

[FrankLeeeee]

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Fixed #3426

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This PR added support for huggingface safetensors, meanwhile, the checkpoint IO API is refactored for better clarity.

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[github-actions]

The code coverage for the changed files is 69%.

Click me to view the complete report

Name                                                      Stmts   Miss  Cover
-----------------------------------------------------------------------------
colossalai/booster/plugin/torch_ddp_plugin.py                73     15    79%
colossalai/checkpoint_io/__init__.py                          4      0   100%
colossalai/checkpoint_io/checkpoint_io_base.py               67     18    73%
colossalai/checkpoint_io/general_checkpoint_io.py            33      9    73%
colossalai/checkpoint_io/index_file.py                       55     38    31%
colossalai/checkpoint_io/utils.py                            77     34    56%
tests/test_booster/test_plugin/test_torch_ddp_plugin.py      70     14    80%
tests/test_checkpoint_io/test_general_checkpoint_io.py       42      1    98%
-----------------------------------------------------------------------------
TOTAL                                                       421    129    69%