Support Two-Stage CenterPoint

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This is a development requirement documentation of a feature we wish to collaborate with the community.
In this issue, we wish to welcome the community developers to participate in the development of Two-Stage CenterPoint.

Steps

• First have a discussion with the mmdet3d team about the development plan. We can divide the full algorithms into multiple modules and add each module step by step in different PR.
• In each PR, add a sub-module of Two-Stage CenterPoint and add unit tests to test your code. The sub-module in this task can include a data converter, a data pipeline, or a component of the framework.
• Benchmark the performance when all modules have been added. The benchmark can be divided into two stages: first for inference and then for training.
• Prepare and release the models.

Resources

Official code release at https://github.com/tianweiy/CenterPoint

Reviewers

@Tai-Wang

[YuanxianH]

Hello, I'd like to pick up this issue. Although I'm new to CenterPoint, I will try my best.