[Feature] Move 'Install MMCV' to a independent CI item. (#602)

* [Feature] Move 'Install MMCV' to a independent CI item.

* Merge MMCV install into MMSEG dependencies install

* Fix bug of 'Install MMCV'

* Remove duplicate CI items

* Fix torch device

* Split cpu env and gpu env into two CI project

* Fix some mmdet related bugs

* Fix mmcv-full install bug of build_cpu CI project.