[Fix] Fix the calculation error of out_w in MaskedConv2dFunction

[zhouzaida]

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Motivation

Fix the calculation error of out_w in MaskedConv2dFunction and this function can not work in torch1.6 due to / operator. More details at https://app.circleci.com/pipelines/github/open-mmlab/mmcv/1574/workflows/3056e22f-a35d-4a55-995f-e66f3acdcc23/jobs/3077?invite=true#step-107-177.

Modification

mmcv/ops/masked_conv.py

BC-breaking (Optional)

No

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 to create this PR.
• [ ] Pre-commit or linting tools indicated in 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.