Normalize the examples from different tasks so they can be merged.

def normalize_example(self, task, example, encoder, hparams, is_infer):
    """Normalize the examples from different tasks so they can be merged."""
    # Here we use the default function for NLP tasks that makes everything
    # a part of "targets" feature. Override in your subclasses for other uses.
    vocab_offset = encoder.vocab_size + len(self.task_list)
    return normalize_example_nlp(
        task, example, is_infer, self.vocab_type, vocab_offset,
        hparams.multiproblem_max_input_length,
        hparams.multiproblem_max_target_length,
        hparams.multiproblem_fixed_train_length)