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This PR resolves an issue loading in tokenizer_configs where the eos or
bos token is null as in:
[Qwen/Qwen1.5-72B-Chat](https://huggingface.co/Qwen/Qwen1.5-72B-Chat/blob/main/tokenizer_config.json)

resolves:
https://github.com/huggingface/text-generation-inference/issues/1545 and
related to https://github.com/QwenLM/Qwen1.5/issues/162