[Feature] Update New SegFormer models on ADE20K.

[MengzhangLI]

Motivatation

Update new SegFormer models to align with original SegFormer model results.

• Update new models which have better numerical results.
• Supporting URLs of PVTv2 pretrained models in MMSegmentation style, which means users would no longer need to transfer keys of pretrained models, they could use our config files to train smoothly.

[codecov]

Codecov Report

Merging #1705 (e8ad3c3) into master (e8dfa3f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1705   +/-   ##
=======================================
  Coverage   90.24%   90.24%           
=======================================
  Files         142      142           
  Lines        8486     8486           
  Branches     1432     1432           
=======================================
  Hits         7658     7658           
  Misses        588      588           
  Partials      240      240           

Flag	Coverage Δ
unittests	90.24% <ø> (ø)

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