How to create a 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 know that I can create a new dataset class by following the instructions found [here](https://github.com/isl-org/Open3D-ML/blob/master/docs/howtos.md#adding-a-new-dataset), but I would like to avoid cloning the entire repo to my project folder only to implement a new dataset class and use it. Seems like an overkill. Is there any possibility to extend the BaseDataset/BaseDatasetSplit classes without cloning the entire repo? I would expect the BaseDataset/BaseDatasetSplit classes to be exposed when installing the pip package, but this is not the case.