src/transformers/pipelines/fill_mask.py/FillMaskPipeline/preprocess
class FillMaskPipeline:    def preprocess(self, inputs, return_tensors=None, **preprocess_parameters) -> Dict[str, GenericTensor]:
        if return_tensors is None:
            return_tensors = self.framework
        model_inputs = self.tokenizer(inputs, return_tensors=return_tensors)
        self.ensure_exactly_one_mask_token(model_inputs)
        return model_inputs