* generate and plot confusion matrix
* fix typo
* add usage and examples for confusion matrix
* deal with nan values(pick pr#7147 mmdet)
* fix md format
1. Fix img path typo in `useful_tools.md`, `zh_cn/model_zoo.md`, and `zh_cn/train.md`
2. Add missing content in `zh_cn/useful_tools.md` to to match `en/useful_tools.md`