[colossalai/checkpoint_io/...] fix bug in load_state_dict_into_model; format error msg

[flymin]

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🚨 Issue number

fix #6019

📝 What does this PR do?

1. resolve issue: will raise an error on any unexpected_keys in strict loading
2. reformat the error message. The current code seems to expect a list rather than a string.

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• [ ] I have added thorough tests.
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[flymin]

I have run the failed tests locally and it did not trigger any error.

[ver217]

We've updated environment of CI. Could you rebase the main branch and rerun the tests?

[flymin]

I rebase the change onto d38344