class_docstr: Sequence to Sequence Dataset in memory.
class_docstr_tok: ["Sequence","to","Sequence","Dataset","in","memory","."]
code: def _check_files_exist(self):
      if not os.path.exists(self.src_file_name):
          raise FileNotFoundError(
              f"Source file {self.src_file_name} not found"
          )
      if not os.path.exists(self.tgt_file_name):
          raise FileNotFoundError(
              f"Source file {self.src_file_name} not found"
          )
  
imports: import os
  import numpy
  import torch
  from nemo.collections.common.tokenizers.tokenizer_spec import TokenizerSpec
  from nemo.collections.nlp.data.language_modeling.megatron.dataset_utils import get_indexed_dataset_, get_samples_mapping
  from nemo.collections.nlp.data.language_modeling.text_memmap_dataset import TextMemMapDataset
  from nemo.core.classes import Dataset
  from nemo.utils import logging
label_desc: Use image node_id 2 for calling the SequenceToSequenceDataset obj's underlying member method code with example usage: obj._check_files_exist() without return types