[Enhance] Add step momentum updater to support MMDet3D

[Wuziyi616]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Similar to StepLRUpdaterHook, in 3D point cloud tasks, we also need to update the momentum of optimizers in a step manner.

Modification

Add a StepMomentumUpdaterHook class.

BC-breaking (Optional)

No.

Use cases (Optional)

Similar to StepLrUpdaterHook.

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. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #1011 (0665222) into master (db6b054) will increase coverage by 0.14%.
The diff coverage is 95.00%.

❗ Current head 0665222 differs from pull request most recent head 0087cac. Consider uploading reports for the commit 0087cac to get more accurate results

@@            Coverage Diff             @@
##           master    #1011      +/-   ##
==========================================
+ Coverage   64.57%   64.71%   +0.14%     
==========================================
  Files         152      152              
  Lines        9792     9843      +51     
  Branches     1779     1791      +12     
==========================================
+ Hits         6323     6370      +47     
- Misses       3141     3144       +3     
- Partials      328      329       +1     

Flag	Coverage Δ
unittests	64.71% <95.00%> (+0.14%)	⬆️

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

Impacted Files	Coverage Δ
mmcv/runner/hooks/momentum_updater.py	64.09% <93.33%> (+3.48%)	⬆️
mmcv/runner/hooks/lr_updater.py	70.19% <100.00%> (+1.03%)	⬆️
mmcv/utils/testing.py	96.00% <0.00%> (-4.00%)	⬇️
mmcv/ops/corner_pool.py	51.35% <0.00%> (-1.43%)	⬇️
mmcv/ops/nms.py	45.65% <0.00%> (ø)
mmcv/utils/__init__.py	88.23% <0.00%> (ø)
mmcv/ops/deform_conv.py	61.76% <0.00%> (+0.28%)	⬆️
mmcv/runner/iter_based_runner.py	56.83% <0.00%> (+0.71%)	⬆️
mmcv/onnx/symbolic.py	41.37% <0.00%> (+0.77%)	⬆️
... and 1 more

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[zhouzaida]

LGTM