Do not require the gitlab-runner to have an available GPU

The unit tests don't need any GPUs. However, it seems that the pin_memory operation (which copies data from the CPU to a GPU) is currently always done, regardless of the usage of a GPU. We should instead disable this parameter when

• no GPU is required
• no GPU is available

The first step could be easy, we can simply disable it from the DAN configuration. The second one is harder, we could look at the list of processes with torch.cuda.list_gpu_process for instance...