[fix] add type hints for yolo_neck

[JunyaoHu]

Motivation

超级视客营mmdet基础任务，选择链接中任一part代码添加type hints，为part-9的yolo_neck.py添加type hints

Modification

为yolo_neck.py中的DetectionBlock和YOLOV3Neck相关函数添加type hints

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

[CLAassistant]

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