[nfc] fix : CheckpointIndexFile is not defined

[digger-yu]

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📝 What does this PR do?

fix "CheckpointIndexFile" is not defined"
modified: colossalai/checkpoint_io/utils.py line 371

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[digger-yu]

@Fridge003 @FrankLeeeee
in #4142 Modified were deleted
In the latest version there will still be this prompt
""CheckpointIndexFile" is not defined"
am i wrong ?

[Fridge003]

There will be a circular importing error if adding from .index_file import CheckpointIndexFile.

[digger-yu]

@Fridge003 in this file is there "from colossalai.checkpoint_io import CheckpointIndexFile" is better or not