fix doctest issues

def get_default_tokenizer():

    default_vocab_path = (

        pkg_resources.resource_filename(

            "chemberta",

            "tokenizers/vocab.txt"

            "deepchem",

            "feat/tests/vocab.txt"

        )

    )

    return SmilesTokenizer(default_vocab_path)

    >>> from deepchem.feat.smiles_tokenizer import SmilesTokenizer

    >>> current_dir = os.path.dirname(os.path.realpath(__file__))

    >>> vocab_path = os.path.join(current_dir, 'data', 'vocab.txt')

    >>> vocab_path = os.path.join(current_dir, 'tests/data', 'vocab.txt')

    >>> tokenizer = SmilesTokenizer(vocab_path)

    >>> print(tokenizer.encode("CCC(CC)COC(=O)[C@H](C)N[P@](=O)(OC[C@H]1O[C@](C#N)([C@H](O)[C@@H]1O)C1=CC=C2N1N=CN=C2N)OC1=CC=CC=C1"))

    [12, 16, 16, 16, 17, 16, 16, 18, 16, 19, 16, 17, 22, 19, 18, 33, 17, 16, 18, 23, 181, 17, 22, 19, 18, 17, 19, 16, 33, 20, 19, 55, 17, 16, 23, 18, 17, 33, 17, 19, 18, 35, 20, 19, 18, 16, 20, 22, 16, 16, 22, 16, 21, 23, 20, 23, 22, 16, 23, 22, 16, 21, 23, 18, 19, 16, 20, 22, 16, 16, 22, 16, 16, 22, 16, 20, 13]

    References