[api] implemented the checkpoint io module

[FrankLeeeee]

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Fixed #3053

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This PR added the checkpoint IO module for abstraction of the loading/saving logic for training and inference. Both unsharded/sharded checkpoint is supported. A test is provided to test the unsharded checkpoint saving/loading.

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[github-actions]

The code coverage for the changed files is 63%.

Click me to view the complete report

Name                                                     Stmts   Miss  Cover
----------------------------------------------------------------------------
colossalai/checkpoint_io/__init__.py                         3      0   100%
colossalai/checkpoint_io/checkpoint_io_base.py              95     54    43%
colossalai/checkpoint_io/general_checkpoint_io.py           35      8    77%
tests/test_checkpoint_io/test_general_checkpoint_io.py      36      1    97%
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TOTAL                                                      169     63    63%