use warning instead of error

except ModuleNotFoundError:

  pass

try:

  from transformers import RobertaTokenizerFast

except ModuleNotFoundError:

  # raise ImportError('This class requires transformers.')

  raise ImportWarning(

      'This class requires transformers. Please use "pip install transformers" to continue.'

  )

  pass

from deepchem.feat.roberta_tokenizer import RobertaFeaturizer

# support classes

    """

    # the encoding is natively a dictionary with keys 'input_ids' and 'attention_mask'

    # -> make this a list of two lists to allow np to handle it

    # encoding = list(self(smiles_string, **kwargs).values())

    encoding = list(self(sequence, **kwargs).values())

    return encoding