Include Argoverse2 data preprocessing (open-mmlab#1321)

Author: yukang2017

docs/GETTING_STARTED.md

@@ -108,6 +108,37 @@ python -m pcdet.datasets.waymo.waymo_dataset --func create_waymo_infos \
 
 Note that you do not need to install `waymo-open-dataset` if you have already processed the data before and do not need to evaluate with official Waymo Metrics. 
 
+### Argoverse2 Dataset
+* Download the **Argoverse 2 Sensor Dataset** from the [official website](https://www.argoverse.org/av2.html#download-link), and then extract them.
+* Install the official API of Argoverse 2
+```shell
+pip install av2==0.2.0
+```
+* Generate info files for `train` and `val`. 
+```python
+python -m pcdet.datasets.argo2.argo2_dataset --root_path data/argo2/sensor --output_dir data/argo2
+```
+- Note that this [issue](https://github.com/argoverse/av2-api/issues/102) from the argo2 api might be noticed. 
+- If the CPU memory of your machine is limited, you can set `--workers=0` in the training script.
+- The organized files are as follows:
+```
+OpenPCDet
+├── data
+│   ├── argo2
+│   │   │── ImageSets
+│   │   │   ├──train.txt & val.txt
+│   │   │── training
+│   │   │   ├──velodyne
+│   │   │── sensor
+│   │   │   ├──val
+│   │   │── argo2_infos_train.pkl
+│   │   │── argo2_infos_val.pkl
+│   │   │── val_anno.feather
+
+├── pcdet
+├── tools
+```
+
 
 ### ONCE Dataset
 * Please download train/val/test of the official [ONCE Dataset](https://once-for-auto-driving.github.io/download.html#downloads) and