code: def _aux_data(self, i):
          
          self.wait_to_read()
          hdl = NDArrayHandle()
          check_call(_LIB.MXNDArrayGetAuxNDArray(self.handle, i, ctypes.byref(hdl)))
          return NDArray(hdl)
docs: Get a deep copy NDArray of the i-th aux data array associated with the
          BaseSparseNDArray.
  
          This function blocks. Do not use it in performance critical code.