src/transformers/data/data_collator.py/DataCollatorMixin/__call__
class DataCollatorMixin:    def __call__(self, features, return_tensors=None):
        if return_tensors is None:
            return_tensors = self.return_tensors
        if return_tensors == "tf":
            return self.tf_call(features)
        elif return_tensors == "pt":
            return self.torch_call(features)
        elif return_tensors == "np":
            return self.numpy_call(features)
        else:
            raise ValueError(f"Framework '{return_tensors}' not recognized!")