[Draft] New S2ANet Implment with cascade

[liuyanyi]

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Motivation

A draft s2anet implment with cascade support. #274 #112

Modification

Add CascadeS2AHead. Add Registry for align module. Add configs for new s2anet and cascade s2anet.

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos? 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

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov[bot]]

Codecov Report

Merging #294 (8ecdd9c) into dev (1828f38) will increase coverage by 0.01%. The diff coverage is 25.77%.

@@            Coverage Diff             @@
##              dev     #294      +/-   ##
==========================================
+ Coverage   31.52%   31.53%   +0.01%     
==========================================
  Files         113      116       +3     
  Lines        7379     7494     +115     
  Branches     1138     1147       +9     
==========================================
+ Hits         2326     2363      +37     
- Misses       4946     5029      +83     
+ Partials      107      102       -5     

Flag	Coverage Δ
unittests	31.50% <25.77%> (+0.02%)	:arrow_up:

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

Impacted Files	Coverage Δ
mmrotate/models/utils/orconv.py	25.42% <ø> (ø)
mmrotate/models/dense_heads/odm_refine_head.py	22.61% <13.63%> (-3.19%)	:arrow_down:
mmrotate/models/dense_heads/cascade_s2a_head.py	18.07% <18.07%> (ø)
mmrotate/models/utils/align_module.py	30.13% <30.13%> (ø)
mmrotate/models/detectors/cascade_s2anet.py	53.84% <53.84%> (ø)
mmrotate/models/dense_heads/__init__.py	100.00% <100.00%> (ø)
mmrotate/models/detectors/__init__.py	100.00% <100.00%> (ø)
mmrotate/models/utils/__init__.py	100.00% <100.00%> (ø)
mmrotate/models/dense_heads/rotated_rpn_head.py	12.29% <0.00%> (-1.24%)	:arrow_down:

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