Support Nuscenes LiDAR Seg dataset

[ZCMax]

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 support of Nuscenes LiDAR Seg dataset.

Steps

• Implement the NuScenesSegDataset
• Add unit tests to test your code
• Add Nuscenes Dataset for 3D Semnantic Segmentation introduction in doc

Resources

• https://www.nuscenes.org/lidar-segmentation