[Feature] Add windows CI

[ZwwWayne]

Add Windows CI in GitHub actions.

[codecov]

Codecov Report

Merging #1023 (010f398) into master (db580dd) will increase coverage by 0.43%.
The diff coverage is 64.28%.

❗ Current head 010f398 differs from pull request most recent head 0a98e15. Consider uploading reports for the commit 0a98e15 to get more accurate results

@@            Coverage Diff             @@
##           master    #1023      +/-   ##
==========================================
+ Coverage   67.99%   68.43%   +0.43%     
==========================================
  Files         159      160       +1     
  Lines       10426    10612     +186     
  Branches     1897     1939      +42     
==========================================
+ Hits         7089     7262     +173     
- Misses       2968     2975       +7     
- Partials      369      375       +6     

Flag	Coverage Δ
unittests	68.43% <64.28%> (+0.43%)	⬆️

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

Impacted Files	Coverage Δ
mmcv/__init__.py	100.00% <ø> (ø)
mmcv/arraymisc/__init__.py	100.00% <ø> (ø)
mmcv/arraymisc/quantization.py	100.00% <ø> (ø)
mmcv/cnn/__init__.py	100.00% <ø> (ø)
mmcv/cnn/alexnet.py	26.08% <ø> (ø)
mmcv/cnn/bricks/__init__.py	100.00% <ø> (ø)
mmcv/cnn/bricks/context_block.py	96.96% <ø> (ø)
mmcv/cnn/bricks/conv.py	100.00% <ø> (ø)
mmcv/cnn/bricks/conv2d_adaptive_padding.py	94.73% <ø> (ø)
mmcv/cnn/bricks/conv_module.py	100.00% <ø> (ø)
... and 107 more

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[fcakyon]

@ZwwWayne how is this pr going? is it related with prebuilt binaries for windows?

[ZwwWayne]

@ZwwWayne how is this pr going? is it related with prebuilt binaries for windows?

Yes, we plan to merge it in the next release. The next step is to release prebuilt binaries for windows.

[ZwwWayne]

LGTM, invite @innerlee to have a look.