Internal function to validate the transformer before applying all internal transformers.

def _validate(self):
        '''
        Internal function to validate the transformer before applying all internal transformers.
        '''
        if self.f_labels is None:
            raise NotFittedError('FeatureRepMix')

        if not self.transformers:
            return

        names, transformers, _ = zip(*self.transformers)

        # validate names
        self._validate_names(names)

        # validate transformers
        for trans in transformers:
            if not isinstance(trans, FeatureRep):
                raise TypeError("All transformers must be an instance of FeatureRep."
                                " '%s' (type %s) doesn't." % (trans, type(trans)))