[WIP] Move some predict code to BaseAngleDenseHead

[liuyanyi]

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Motivation

Rotated FCOS and RTMDet share same code in predict, move them to a base class. Future model with angle pred can use this base class too.

TODO:

• Share code with AngleBranchRetinaHead
• More comments and fix some words.
• Test acc.

Modification

Please briefly describe what modification is made in this PR.

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)

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

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