Dev 1.x Adapt PETR to mmcv v2 version dataset

[WendellZ524]

Motivation

I am trying to run PETR project through mmdetction3d, but I found that the implementation is obsoleted

Modification

I updated the document of PETR project to quickly run a training and testing script.

And I updated the PETR head to adapt to the v2 dataset format.

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.

[CLAassistant]

All committers have signed the CLA.

[Xiangxu-0103]

Hi@WendellZ524, thanks for your contribution. Please sign the CLA first so that we can merge this PR.