[Docs] Add Tpvformer readme

[sunjiahao1999]

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Motivation

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

Modification

Please briefly describe what modification is made in this PR.

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

Patch coverage: 42.30% and project coverage change: -0.05 ⚠️

Comparison is base (d99dbce) 47.03% compared to head (90b6025) 46.99%.

❗ Current head 90b6025 differs from pull request most recent head 08b16be. Consider uploading reports for the commit 08b16be to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2517      +/-   ##
===========================================
- Coverage    47.03%   46.99%   -0.05%     
===========================================
  Files          276      276              
  Lines        23111    23144      +33     
  Branches      3602     3611       +9     
===========================================
+ Hits         10871    10877       +6     
- Misses       11554    11579      +25     
- Partials       686      688       +2     

Flag	Coverage Δ
unittests	46.99% <42.30%> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
mmdet3d/datasets/kitti_dataset.py	55.10% <ø> (ø)
mmdet3d/datasets/lyft_dataset.py	78.26% <ø> (ø)
mmdet3d/datasets/nuscenes_dataset.py	47.61% <ø> (ø)
mmdet3d/datasets/s3dis_dataset.py	78.08% <ø> (ø)
mmdet3d/datasets/scannet_dataset.py	82.75% <ø> (ø)
mmdet3d/datasets/sunrgbd_dataset.py	69.44% <ø> (ø)
mmdet3d/apis/inference.py	12.97% <10.00%> (-0.67%)	⬇️
mmdet3d/visualization/local_visualizer.py	30.13% <33.33%> (-0.19%)	⬇️
mmdet3d/datasets/seg3d_dataset.py	71.42% <60.00%> (-0.74%)	⬇️
mmdet3d/datasets/det3d_dataset.py	68.55% <88.88%> (-0.44%)	⬇️
... and 1 more

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[cuge1995]

@JingweiZhang12 There is [Bug] Error while evaluating TPVFormer #2885
See #2885