Make accelerator-memory resource and logic optional

[tardieu]

The accelerator-memory resource InstaSlice generates makes it possible to implement fungible Kubernetes quotas, i.e., define MIG slice quotas in terms of accelerator memory instead of MIG slice profiles. While this is a very useful feature, this will not be necessary when relying on Kueue quotas or when not using quotas at all. We could therefore make the resource and logic optional and opt in.