Pointpillars parameter for custom dataset.

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D-ML/issues).
- [X] I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

I've developed a custom data loader for a custom dataset of 3 classes I have, it does not have any calibration or image files and simply contains lidar data and 3D bbox annotations. (I also made sure that the bbox annotations have the correct values by using a KITTI visualization tool on my own dataset and making sure that the bboxes are correctly placed).

I've trained several iterations of Pointpillars with different configuration inputs but I'm not getting good results even after 100-150 epochs.

Each pointcloud is around 150k points,
I believe that feature extraction config part of the model is the most crucial section in the config file:

--------------------------------------------------------
voxelize:
max_num_points: 100 # Max number of point per pillar
voxel_size: &vsize
[0.25, 0.25, 4]
max_voxels: [30000, 40000] # Number of pillars for Training,Testing

voxel_encoder:
in_channels: 3
feat_channels: [64, 64]
voxel_size: *vsize

scatter:
in_channels: 64
output_shape: [400, 400]

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yet I'm not sure how to choose the most adequate values.
Any idea on how to go about it?