[workflow] fixed the post-commit failure when no formatting needed

[FrankLeeeee]

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Fixed #3021

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This PR fixed the post-commit workflow such that it is still seen as successful if all files are well-formatted and there is nothing to commit. Have tested the new workflow file in https://github.com/hpcaitech/ColossalAI/actions/runs/4344828018.

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