code: def merge_file_(self, another_file):
      # Concatenate index
      index = MMapIndexedDataset.Index(index_file_path(another_file))
      assert index.dtype == self._dtype
  
      for size in index.sizes:
          self._sizes.append(size)
  
      # Concatenate data
      with open(data_file_path(another_file), "rb") as f:
          shutil.copyfileobj(f, self._data_file)
  
imports: import os
  import shutil
  import struct
  from functools import lru_cache
  from itertools import accumulate
  import numpy
  import torch
label_desc: Use image node_id 4 for calling the MMapIndexedDatasetBuilder obj's underlying member method code with example usage: obj.merge_file_(another_file) without return types