Saves the current epoch rollouts to disk, split into train/dev sets.

def generate_data(self, data_dir, tmp_dir=None, task_id=-1):
    """Saves the current epoch rollouts to disk, split into train/dev sets."""
    if not self._rollouts_by_epoch_and_split[self.current_epoch]:
      # Data not loaded from disk.
      self._split_current_epoch()

    rollouts_by_split = self._rollouts_by_epoch_and_split[self.current_epoch]
    splits_and_paths = self.splits_and_paths(data_dir)

    for (split, paths) in splits_and_paths:
      rollouts = rollouts_by_split[split]
      num_frames = self._calc_num_frames(rollouts)
      shard_size = num_frames // len(paths)

      frame_gen = self._generate_frames(rollouts)
      for (path_index, path) in enumerate(paths):
        limit = shard_size
        # Put the remainder in the last shard to preserve the ordering.
        if path_index == len(paths) - 1:
          limit = None
        generator_utils.generate_files(
            itertools.islice(frame_gen, limit), [path],
            cycle_every_n=float("inf")
        )