[Fix] Fix the bug of counting ignore index in IOU in seg_eval

[xizaoqu]

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Motivation

seg_eval.py will don't filter out ignore index when counting mIOU. This bug is because it assumes that predictions will not include ignore_index.

Modification

Set IOU with ignore_index as nan to filter it out.

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos?
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)

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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.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[jason102811]

Dear xizaoqu,
First of all, we want to express our gratitude for your significant PR in the MMDet project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.
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Best regards！ @xizaoqu