Yield dicts for Text2TextProblem.generate_samples from lines of files.

def text2text_distill_iterator(source_txt_path, target_txt_path,
                               distill_txt_path):
  """Yield dicts for Text2TextProblem.generate_samples from lines of files."""
  for inputs, targets, dist_targets in zip(
      txt_line_iterator(source_txt_path), txt_line_iterator(target_txt_path),
      txt_line_iterator(distill_txt_path)):
    yield {"inputs": inputs, "targets": targets, "dist_targets": dist_targets}