[Feature] Support MaskFormer(NeurIPS'2021) in MMSeg 1.x (#2215)

* [Feature] Support MaskFormer(NeurIPS'2021) in MMSeg 1.x

* add mmdet try except logic

* refactor config files

* add readme

* fix config

* update models & logs

* add MMDET installation and fix info

* fix comments

* fix

* fix config norm optimizer setting

* update models & logs & unittest

* add docstring of MaskFormerHead

* wait for mmdet 3.0.0rc4

* replace seg_mask with seg_logits & add docstring for batch_input_shape

* use mmdet3.0.0rc4

* fix readme and modify config comments

* add mmdet installation in pr_stage_test.yml

* update mmcv version in pr_stage_test.yml

* add mmdet in build_cpu of pr_stage_test.yml

* modify mmdet& mmcv installation in merge_stage_test.yml

* fix typo

* update test.yml

* update test.yml