Keep reference to GPU from Task after use

Refs https://gitlab.com/teklia/requests/-/issues/829#note_1355141432

We want to know which GPU was used during a task execution once it finished.

The idea is to use an FK instead of 1-to-1, keep it populated and introduce a constraint on gpu_id + state

So we can update the existing OneToOneField to a ForeignKey, which does not require a data migration, and we can add the unique constraint with UNIQUE (gpu_id) WHERE (state IN ('unscheduled', 'pending', 'running'))