[Enhancement] Delete convert function and add instruction to ViT/Swin README.md

[xiexinch]

Modification

1. Delete convert function from ViT/Swin backbones.
2. Add instructions to README.md

BC-breaking

Directly use other repositories' model links to start a train may not work, users must use convert scripts (#783 ) to preprocess pre-trained weights.

[codecov]

Codecov Report

Merging #791 (9737d89) into master (f72727c) will increase coverage by 2.63%.
The diff coverage is 79.27%.

❗ Current head 9737d89 differs from pull request most recent head cbf309c. Consider uploading reports for the commit cbf309c to get more accurate results

@@            Coverage Diff             @@
##           master     #791      +/-   ##
==========================================
+ Coverage   86.09%   88.73%   +2.63%     
==========================================
  Files         108      107       -1     
  Lines        5862     5805      -57     
  Branches      953      932      -21     
==========================================
+ Hits         5047     5151     +104     
+ Misses        630      462     -168     
- Partials      185      192       +7     

Flag	Coverage Δ
unittests	88.71% <79.27%> (+2.63%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmseg/models/utils/__init__.py	100.00% <ø> (ø)
mmseg/apis/test.py	46.34% <48.27%> (+15.67%)	⬆️
mmseg/datasets/ade.py	90.90% <72.72%> (+50.36%)	⬆️
mmseg/datasets/cityscapes.py	85.00% <75.00%> (+63.65%)	⬆️
mmseg/datasets/custom.py	92.13% <93.33%> (+2.31%)	⬆️
mmseg/core/evaluation/__init__.py	100.00% <100.00%> (ø)
mmseg/core/evaluation/eval_hooks.py	72.54% <100.00%> (+4.36%)	⬆️
mmseg/core/evaluation/metrics.py	90.62% <100.00%> (+1.33%)	⬆️
mmseg/models/backbones/swin.py	87.41% <100.00%> (+0.51%)	⬆️
mmseg/models/backbones/vit.py	86.79% <100.00%> (+1.94%)	⬆️
... and 3 more

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[xvjiarui]

LGTM.
Please resolve the conflict.

[xvjiarui]

The url in config files should be removed.

[Junjun2016]

Unitests are failed.