Multi GPU support

[tomhosking]

Multi GPU support would be useful - take another look at lightning

[tomhosking]

Implemented via lightning.Fabric in d6a74ab. Could do with tidying up:

• ideally use proper Lightning trainers etc
• device mapping should be managed within BaseAgent. This is a little tricky because Fabric needs to modify dataloaders as well as models, which are separate from Agents