fix softmax focal loss algorithm

[qingpeng9802]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

about #2892
fix softmax focal loss algorithm

Modification

same above

BC-breaking (Optional)

The original softmax focal loss algorithm is totally changed.

Use cases (Optional)

No change

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.

[CLAassistant]

All committers have signed the CLA.

[grimoire]

Test case can be added in https://github.com/open-mmlab/mmcv/blob/main/tests/test_ops/test_focal_loss.py

[grimoire]

Since focal loss is widely used in other repos. We better assign more reviewer from these repos @zhouzaida

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[qingpeng9802]

If there are no more comments about the implementation, I will add some tests.

[grimoire]

Since focal loss is widely used in other repos in OpenMMLab. I am contacting maintainers of other repos to review this PR.