[Misc] add return token strs for tokenize

[reidliu41]

FIX #18928 (link existing issues this PR will resolve)

$ curl -X POST http://localhost:8000/tokenize   -H "Content-Type: application/json"   -d '{
    "model": "meta-llama/Meta-Llama-3-8B-Instruct",
    "prompt": "Hello world!",
    "return_token_strs": true
}'|jq

{
  "count": 4,
  "max_model_len": 8192,
  "tokens": [
    128000,
    9906,
    1917,
    0
  ],
  "token_strs": [
    "<|begin_of_text|>",
    "Hello",
    "Ġworld",
    "!"
  ]
}

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[reidliu41]

@DarkLight1337 hi, could you please help to take a look this if you have time? thanks

[DarkLight1337]

Can you add a test?

[DarkLight1337]

Thanks for adding this!