Support for PartNet-Mobility Dataset

[beneisner]

(Similar in theme to #2791, but I created a separate issue so they could be discussed separately)

The PartNet-Mobility Dataset is a 3D dataset that provides a bunch of fine-grained semantic and instance labels for a large set of articulated 3D objects. The dataset contains both meshes and point clouds. Additionally, it contains articulations for each object, which describe how individual parts move in relation to one another, i.e. what direction a drawer slides. It is an extension of the ShapeNet effort, which this pytorch_geometric already supports.

I already have an implementation of this dataset that I'm using in my research, and would be happy to upstream it. Would such a contribution be welcome?

[rusty1s]

It's also very much welcome, although we should probably start with one of the datasets.

[beneisner]

@rusty1s I'm getting around to this now (hopefully). This particular dataset is somewhat complicated and has several different aspects to it; do you tend to require that the entire dataset object fit into a single file in datasets/ ? Or can it span multiple files (i.e. a utils file, etc.)

[rusty1s]

I think it can span multiple files. If you think this is the way to go, please feel free to propose a first version :)