Shuffle a single file of records.

  Args:
    fname: a string
    extra_fn: an optional function from list of TFRecords to list of TFRecords
      to be called after shuffling.

def _shuffle_single(fname, extra_fn=None):
  """Shuffle a single file of records.

  Args:
    fname: a string
    extra_fn: an optional function from list of TFRecords to list of TFRecords
      to be called after shuffling.
  """
  records = read_records(fname)
  random.shuffle(records)
  if extra_fn is not None:
    records = extra_fn(records)
  out_fname = fname.replace(UNSHUFFLED_SUFFIX, "")
  write_records(records, out_fname)
  tf.gfile.Remove(fname)