Re-export JobConfig in lerobot/configs/__init__.py so external callers
import it as `from lerobot.configs import JobConfig`, matching the other
config classes. Adapt the train script and test imports.
Introduce a JobConfig draccus group on TrainPipelineConfig (--job.target/image/
timeout/detach/tags) whose is_remote property gates remote dispatch, plus a
save_checkpoint_to_hub flag and validation. Add push_checkpoint_to_hub(), which
uploads a saved checkpoint directory to the model repo under checkpoints/<step>/
and creates the repo idempotently (private propagates from policy.private).
Nicolas Rabault