class_docstr: Map-reduce way of calculating the sum of a stream of values.
  
  `partial_result` represents one of two things, depending on the axis:
  Case 1 - axis 0 is aggregated (axis is None or 0):
      In each `step`, sum is being calculated and added to `partial_result`.
  
  Case 2 - axis 0 is not being aggregated:
      In this case, `partial_result` is a list that in the end gets
      concatenated to a np.ndarray.
code: def get(self) -> np.ndarray:
      assert self.axis is None or isinstance(self.axis, tuple)
  
      if self.axis is None or 0 in self.axis:
          assert isinstance(self.partial_result, np.ndarray)
          return np.copy(self.partial_result)
  
      assert isinstance(self.partial_result, list)
      return np.concatenate(self.partial_result)
  
file_path: gluonts/src/gluonts/ev/aggregations.py
imports: from dataclasses import dataclass
  from typing import List, Optional, Union
  import numpy
label_desc: Use image node_id 2 for calling the Sum obj's underlying member method code with example usage: obj.get() and returns: np, np