[CI] Add github action CI && refactor tests/utils/

[JingweiZhang12]

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

Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

1. Add github action CI
2. Move tests/utils/*.py from tests to mmdet3d/testing for right importing on Win, like MMDet.

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

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (dev-1.x@4909618). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head fc6cd95 differs from pull request most recent head d6a3d6c. Consider uploading reports for the commit d6a3d6c to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-1.x    #2012   +/-   ##
==========================================
  Coverage           ?   45.41%           
==========================================
  Files              ?      246           
  Lines              ?    19948           
  Branches           ?     3192           
==========================================
  Hits               ?     9060           
  Misses             ?    10277           
  Partials           ?      611           

Flag	Coverage Δ
unittests	45.41% <0.00%> (?)

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

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[ZCMax]

We need to resolve the CUDA out of memory problem in GitHub action CI?

[JingweiZhang12]

We need to resolve the CUDA out of memory problem in GitHub action CI?

I tried to reduce memory footprint several mouths ago. Now, the occupancy is very small, but the RAM on Win env in the new GitHub CI is too small. The fix is a little challenging since we don't have Win env.
I think the success on one Win env can make sure the PR is normal.