Allow checkpointer to immediately preempt and requeue when receiving a signal

[f-dangel]

We could introduce a flag preempt_immediately=False to Checkpointer. If set to True, upon receiving the signal, the checkpointer will immediately mark the run as preempted and requeue the job, without saving a checkpoint. This would be useful for clusters that do not have any guarantee for a preemption-free initial period.

In this case, the responsibility to properly sync the Weights & Biases logs with the logs from the resumed run is up to the user. But we can have an example tutorial to demonstrate how it works (I think one would do this by storing the wandb step in the checkpoint using the extra_info option in .step, which would then be retrieved with .load_latest_checkpoint).