Metadata about GPU requirements

Primitives should probably be able to tell:

• can they use GPUs to run
• do they require GPUs to run
• how many GPUs do they need, if multiple are available?

Moreover, there should be some interface to tell which GPU a primitive should use. Or is this caller's responsibility to mask a particular GPU?

Sidenote: we should also standardize which approach for accessing GPUs do we want to use in our Docker images. How to run that on Kubernetes?