Why is the vocabulary of token_type_ids and input_ids shared? 
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https://github.com/huggingface/transformers/blob/17ea43cf985829634bd86b36b44e5410c6f83e36/transformers/modeling_gpt2.py#L421

In GPT2Model, forward method, it seems the vocabulary of token_type_ids and input_ids is shared. I checked the vocabulary table, 0 and 1 corresponds to the exclamation sign and the quote sign. What is the reason of sharing the vocabulary? Is it on purpose? 

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