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`
* [Enhance] New-style CPU training and inference.
* assert mmcv version
* SyncBN to BN in training and testing
* SyncBN to BN in training and testing
* upload untracked files to this branch
* delete gpu_ids
* fix bugs
* assert args.gpu_id in train.py
* use cfg.gpu_ids = [args.gpu_id]
* use cfg.gpu_ids = [args.gpu_id]
* fix typo
* fix typo
* fix typos