Fix assertion failure during chat session.

[prideout]

This fixes the following assert that is easy to repro in any chat session:

Traceback (most recent call last):
    File "/home/ubuntu/cali/torchchat/torchchat.py", line 69, in <module>
        generate_main(args)
    File "/home/ubuntu/cali/torchchat/generate.py", line 896, in main
        for _ in gen.chat(generator_args):
    File "/home/ubuntu/cali/torchchat/generate.py", line 748, in chat
        self.chat_formatter.encode_header(
    File "/home/ubuntu/cali/torchchat/generate.py", line 53, in encode_header
        tokens.extend(self.tokenizer.encode(role, bos=False, eos=False))
    File "/home/ubuntu/cali/torchchat/tokenizer/tiktoken.py", line 133, in encode
        assert type(s) is str

I believe this regressed with #1035.

[pytorch-bot]

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[Jack-Khuu]

Thanks for flagging and fixing @prideout !