Extracts list of subwords from file.

def load_from_file(cls, filename_prefix):
    """Extracts list of subwords from file."""
    filename = cls._filename(filename_prefix)
    lines, _ = cls._read_lines_from_file(filename)
    # Strip wrapping single quotes
    vocab_list = [line[1:-1] for line in lines]
    return cls(vocab_list=vocab_list)