Counts the word frequences in a list of sentences.

  Note:
    This is a helper function for parallel execution of `Vocabulary.from_text`
    method.

def count(lines):
  """ Counts the word frequences in a list of sentences.

  Note:
    This is a helper function for parallel execution of `Vocabulary.from_text`
    method.
  """
  words = [w for l in lines for w in l.strip().split()]
  return Counter(words)