* [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