fix run chat

[metascroy]

The BPE tokenizer does not pattern match the special SOS and EOS tokens and they must be explicitly added with the encode function's 2nd and 3rd arguments.

Sample output after change:

(cchat2) scroy@scroy-mbp torchchat % ./cmake-out/aoti_run ./model.so -z ./.model-artifacts/meta-llama/Llama-2-7b-chat-hf/tokenizer.bin -m chat -t 0
Failed to load ./.model-artifacts/meta-llama/Llama-2-7b-chat-hf/tokenizer.bin into a Tiktoken tokenizer. Trying sentencepiece tokenizer..
Enter system prompt (optional): Be brief.  Answer in 10 words or less.
User: What is the answer to the universe?
Assistant:   The answer to the universe is 42.
User: Why did you say that?
Assistant:   I'm just joking! The answer to the universe is not actually 42. It's a humorous reference to a popular meme and book series called "The Hitchhiker's Guide to the Galaxy." In the series, the character Deep Thought, a supercomputer, is asked to find the answer to the universe, and after 7.5 million years of calculation, it finally reveals that the answer is 42. However, the character explains that the answer is meaningless and that the true answer is "42!"
User: Interesting.  Who wrote that?
Assistant:   The Hitchhiker's Guide to the Galaxy is a series of books written by Douglas Adams. The series includes five books: "The Hitchhiker's Guide to the Galaxy," "Restaurant at the End of the Un

[mikekgfb]

LGTM - @JacobSzwejbka please review as a second set of eyes since you're our expert on chat!

[mikekgfb]

Thank you! Waiting to get Jacob's feedback too, to ensure that generate --chat and runner are aligned