Computes a minimal Experiment protocol buffer by scanning the runs.

def _compute_experiment_from_runs(self):
    """Computes a minimal Experiment protocol buffer by scanning the runs."""
    hparam_infos = self._compute_hparam_infos()
    if not hparam_infos:
      return None
    metric_infos = self._compute_metric_infos()
    return api_pb2.Experiment(hparam_infos=hparam_infos,
                              metric_infos=metric_infos)