code: def _concat(self, first_data, second_data):
          
          assert len(first_data) == len(
              second_data), 'data source should contain the same size'
          if first_data and second_data:
              return [
                  concat(
                      first_data[x],
                      second_data[x],
                      dim=0
                  ) for x in range(len(first_data))
              ]
          elif (not first_data) and (not second_data):
              return []
          else:
              return [
                  first_data[0] if first_data else second_data[0]
                  for x in range(len(first_data))
              ]
docs: Helper function to concat two NDArrays.