add example of SmilesTokenizer

    Please see https://github.com/huggingface/transformers

    and https://github.com/rxn4chemistry/rxnfp for more details.

    Examples

    --------

    >>> from deepchem.feat.smiles_tokenizer import SmilesTokenizer

    >>> from transformers import RobertaForMaskedLM

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

    >>> vocab_path = os.path.join(current_dir, '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"))

    References

    ----------

    .. [1]  Schwaller, Philippe; Probst, Daniel; Vaucher, Alain C.; Nair, Vishnu H; Kreutter, David;

    Note

    ----

    This class requires huggingface's transformers and tokenizers libraries to be installed.

    """

    """

    def __init__(

            self,

            vocab_file: str='',