* [Doc] Add more clearly statement of CPU training/inference

* Update docs/en/inference.md

Co-authored-by: Miao Zheng <76149310+MeowZheng@users.noreply.github.com>

* Add training startup documentation

* fix

* fix

* fix

* fix

* fix

* fix

* fix

* fix

* fix

* [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

* merge docs/ and docs_zh-CN/

* merge docs/ and docs_zh-CN/

* merge docs/ and docs_zh-CN/

* merge docs/ and docs_zh-CN/

* fix launch utility url

* fix launch utility url

* fix wrong pytorch doc url

* remove wrong links docs//

* [Feature] Change option to cfg-option

* add expire date and fix the docs

* modify docstring

* [Fix] Convert SyncBN to BN when training on DP.

* Modify SyncBN2BN.

* Add SyncBN2BN unit test.

* Resolve some comments.

* use mmcv official revert_sync_batchnorm

* Remove local syncbn2bn unit tests.

* Update mmcv version.

* Fix bugs of gather model tools.

* Modify warnings.

* Modify docker mmcv version.

* Update mmcv version table.

* refactor docs

* add docs

* add modelzoo

* refactor getting started