[Enhance] Add benchmark regression script

[yezhen17]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.
Manually performing benchmark regression is boring and laborious. We need a script to enable automatic regression. Refer to https://github.com/open-mmlab/mmdetection/blob/master/.dev_scripts/train_benchmark.sh

Modification

Please briefly describe what modification is made in this PR.
I added a script to perform automatic benchmark regression, and also the script to generate such kind of scripts (can be easily used to generate benchmarking scripts given the config list to benchmark.

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

Merging #808 (acf84ef) into master (43b4632) will increase coverage by 0.29%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #808      +/-   ##
==========================================
+ Coverage   49.03%   49.33%   +0.29%     
==========================================
  Files         210      210              
  Lines       15964    16021      +57     
  Branches     2547     2556       +9     
==========================================
+ Hits         7828     7904      +76     
+ Misses       7634     7614      -20     
- Partials      502      503       +1     

Flag	Coverage Δ
unittests	49.33% <ø> (+0.29%)	⬆️

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

Impacted Files	Coverage Δ
mmdet3d/core/bbox/structures/utils.py	80.51% <0.00%> (-16.63%)	⬇️
mmdet3d/datasets/s3dis_dataset.py	88.57% <0.00%> (-6.10%)	⬇️
mmdet3d/apis/inference.py	43.21% <0.00%> (-0.85%)	⬇️
mmdet3d/models/fusion_layers/point_fusion.py	64.44% <0.00%> (-0.78%)	⬇️
mmdet3d/datasets/nuscenes_mono_dataset.py	37.08% <0.00%> (-0.39%)	⬇️
mmdet3d/models/dense_heads/centerpoint_head.py	44.55% <0.00%> (-0.19%)	⬇️
mmdet3d/__init__.py	81.48% <0.00%> (ø)
mmdet3d/apis/test.py	15.78% <0.00%> (ø)
mmdet3d/apis/train.py	50.00% <0.00%> (ø)
mmdet3d/ops/__init__.py	100.00% <0.00%> (ø)
... and 164 more

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