[Fix] Fix demo scripts (#1815)

* [Feature] Add SegVisualizer

* change name to visualizer_example

* fix inference api

* fix video demo and refine inference api

* fix

* mmseg compose

* set default device to cuda:0

* fix import

* update dir

* rm engine/visualizer ut

* refine inference api and docs

* rename

Co-authored-by: MengzhangLI <mcmong@pku.edu.cn>