<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def submit(cls, job_config, in_xg_transaction=False):
 """Submit the job to run. Args: job_config: an instance of map_job.MapJobConfig. in_xg_transaction: controls what transaction scope to use to start this MR job. If True, there has to be an already opened cross-group transaction scope. MR will use one entity group from it. If False, MR will create an independent transaction to start the job regardless of any existing transaction scopes. Returns: a Job instance representing the submitted job. """

    cls.__validate_job_config(job_config)
    mapper_spec = job_config._get_mapper_spec()

    # Create mr spec.
    mapreduce_params = job_config._get_mr_params()
    mapreduce_spec = model.MapreduceSpec(
        job_config.job_name,
        job_config.job_id,
        mapper_spec.to_json(),
        mapreduce_params,
        util._obj_to_path(job_config._hooks_cls))

    # Save states and enqueue task.
    if in_xg_transaction:
      propagation = db.MANDATORY
    else:
      propagation = db.INDEPENDENT

    state = None
    @db.transactional(propagation=propagation)
    def _txn():
      state = cls.__create_and_save_state(job_config, mapreduce_spec)
      cls.__add_kickoff_task(job_config, mapreduce_spec)
      return state

    state = _txn()
    return cls(state)