Save the vocabulary to a file.

def save_to_file(self, filename_prefix):
    """Save the vocabulary to a file."""
    # Wrap in single quotes to make it easier to see the full subword when
    # it has spaces and make it easier to search with ctrl+f.
    filename = self._filename(filename_prefix)
    lines = ["'%s'" % s for s in self._subwords]
    self._write_lines_to_file(filename, lines)