[Fix] Fix the resume of iteration

[LZHgrla]

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Motivation

IterBasedTrainLoop is not correct for resume. The beginning data index is 0, instead of self._iter

Modification

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BC-breaking (Optional)

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Use cases (Optional)

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Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDetection or MMPretrain.
4. The documentation has been modified accordingly, like docstring or example tutorials.