docstr: Export rolling result in form of a dict of numpy ndarray {'x': ..., 'y': ...}
  
  :return: a 2-element dict xshard. each value is a 3d numpy ndarray. The ndarray
           is casted to float32.
code: def to_xshards(self):
      """
      Export rolling result in form of a dict of numpy ndarray {'x': ..., 'y': ...}
  
      :return: a 2-element dict xshard. each value is a 3d numpy ndarray. The ndarray
               is casted to float32.
      """
      if self.numpy_shards is None:
          raise RuntimeError(
              'Please call "roll" method\
                      before transform a XshardsTSDataset to numpy ndarray!'
          )
      return self.numpy_shards.transform_shard(transform_to_dict)
  
file_path: analytics-zoo/pyzoo/zoo/chronos/data/experimental/xshards_tsdataset.py
imports: from zoo.orca.data.shard import SparkXShards
  from zoo.chronos.data.utils.utils import _to_list, _check_type
  from zoo.chronos.data.utils.roll import roll_timeseries_dataframe
  from zoo.chronos.data.utils.split import split_timeseries_dataframe
  from zoo.chronos.data.experimental.utils import add_row, transform_to_dict
label_desc: Use image node_id 4 for calling the XShardsTSDataset obj's underlying member method code with example usage: obj.to_xshards() and returns: self