CodeCamp #34 add type hints for bfp and channel_mapper

[JunyaoHu]

Motivation

超级视客营 mmdet 基础任务，为part-9的 bfp.py 和 channel_mapper.py 添加 type hints

Modification

1. 昨天修改好了 bfp.py 并提交PR且已经过审，但不太熟悉PR操作，没有签署 commit sign 导致无法合并
2. 为 channel_mapper.py 中相关函数添加 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.

[BIGWangYuDong]

LGTM