code: def concat_generator(filename, up_threshold, low_threshold=10):
      """Generate concatenated lines from file upto up_threshold characters."""
      txt = ""
      for line in tf.gfile.Open(filename):
          line = line.strip()
          if len(txt) + len(line) + 1 >= up_threshold:
              ret = txt
              txt = ""
              # We don't yield very short long parts to prevent noisy examples.
              if len(ret) > low_threshold and len(ret) < up_threshold:
                  yield {"targets": ret}
  
          if not txt:
              txt = line
          else:
              txt = " ".join([txt, line])
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import os
  import six
  from tensor2tensor.data_generators import generator_utils
  from tensor2tensor.data_generators import problem
  from tensor2tensor.data_generators import text_problems
  from tensor2tensor.utils import registry
  import tensorflow.compat.v1
label_desc: Use image node_id 1 for calling a global function with example usage: concat_generator(filename, up_threshold, low_threshold) without return types