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## Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

## Modification

Please briefly describe what modification is made in this PR.

## BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

## Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

## Checklist

**Before PR**:

- [ ] I have read and followed the workflow indicated in the [CONTRIBUTING.md](https://github.com/open-mmlab/mmcv/blob/master/CONTRIBUTING.md) to create this PR.
- [ ] Pre-commit or linting tools indicated in [CONTRIBUTING.md](https://github.com/open-mmlab/mmcv/blob/master/CONTRIBUTING.md) are used to fix the potential lint issues.
- [ ] Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
- [ ] New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
- [ ] The documentation has been modified accordingly, including docstring or example tutorials.

**After PR**:

- [ ] If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
- [ ] CLA has been signed and all committers have signed the CLA in this PR.
