fix: #8612
For checkpoint experiment in some case users might want to give it a early
stopping to reduce variance. But currently if we interrupt/kill the
experiment it will be marked as failed, and all of the completed
checkpoints will be removed as we cleanup all the running directly after
the process failed.
1. We raise CheckpointKilledError intead of StageCmdFailed error if at
least one checkpoint had been commited.
2. Temp_dir executor will continue collecting data if the Checkpoint
stage was interrupted.
3. Raise warnings if a checkpoint stage was incomplete and the other
stages were not forwarded.
4. Add a new functional test for this
karajan1001
08ae68caf4