[Docs] Use PyTorch sphinx theme

[zhouzaida]

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Motivation

Use PyTorch sphinx theme

Modification

• docs
• docs_zh_CN

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #1321 (a844ee6) into master (9016fef) will increase coverage by 0.32%.
The diff coverage is 92.72%.

❗ Current head a844ee6 differs from pull request most recent head cae1614. Consider uploading reports for the commit cae1614 to get more accurate results

@@            Coverage Diff             @@
##           master    #1321      +/-   ##
==========================================
+ Coverage   68.81%   69.14%   +0.32%     
==========================================
  Files         161      162       +1     
  Lines       10742    10746       +4     
  Branches     1972     1978       +6     
==========================================
+ Hits         7392     7430      +38     
+ Misses       2962     2927      -35     
- Partials      388      389       +1     

Flag	Coverage Δ
unittests	69.14% <92.72%> (+0.32%)	⬆️

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

Impacted Files	Coverage Δ
mmcv/cnn/utils/sync_bn.py	84.61% <84.61%> (ø)
mmcv/cnn/bricks/conv_module.py	100.00% <100.00%> (ø)
mmcv/cnn/utils/__init__.py	100.00% <100.00%> (ø)
mmcv/ops/cc_attention.py	90.90% <100.00%> (+52.19%)	⬆️
mmcv/runner/checkpoint.py	74.41% <100.00%> (+0.94%)	⬆️
mmcv/utils/config.py	90.12% <100.00%> (+0.02%)	⬆️
mmcv/version.py	82.35% <100.00%> (ø)
mmcv/cnn/utils/weight_init.py	85.92% <0.00%> (+0.36%)	⬆️

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