Implement PackedGraph.to()

[cthoyt]

This pull request implements a to() function on the PackedGraph class. Many models expect a function like this available for shuttling between CPU and GPU.

• Handle when a str is passed (done in bd3482b)
• Handle when a torch.device is passed (done in 84aa285)

Question: should it be able to handle other things?

[KiddoZhu]

That's a good feature! I feel it might not be robust to parse the string in that way. For example, in multi-GPU training mode, one may want to specify which GPU they want to send the data structure to.

A safer way is to convert the string (or whatever argument) into torch.device and then dispatch it with data.Graph.cpu or data.Graph.cuda. I will refine it.

[cthoyt]

It’s definitely not the goal to implement a robust string parser - just to make it possible to do a few common things very quickly. Note that the function also handles torch devices directly as well in the next part of the conditional