Fix index in ChatCompletionChunk

[Wauplin]

Fix a small inconsistency compared the OpenAI's chat-completion behavior (introduced in #1427 cc @drbh). When using stream=True, each chunk has an index value in ChatCompletionChoice. This index is not meant to be the index of the generated token but the index of the choice, which is always 0 (since TGI always return a single choice).

See https://platform.openai.com/docs/api-reference/chat/object:

index integer
The index of the choice in the list of choices.

---

So instead of

data:{"id":"","object":"text_completion","created":1710508199,"model":"HuggingFaceH4/zephyr-7b-beta","system_fingerprint":"1.4.3-sha-e6bb3ff","choices":[{"index":1,"delta":{"role":"assistant","content":"I"},"logprobs":null,"finish_reason":null}]}
data:{"id":"","object":"text_completion","created":1710508199,"model":"HuggingFaceH4/zephyr-7b-beta","system_fingerprint":"1.4.3-sha-e6bb3ff","choices":[{"index":2,"delta":{"role":"assistant","content":"'"},"logprobs":null,"finish_reason":null}]}
data:{"id":"","object":"text_completion","created":1710508199,"model":"HuggingFaceH4/zephyr-7b-beta","system_fingerprint":"1.4.3-sha-e6bb3ff","choices":[{"index":3,"delta":{"role":"assistant","content":"m"},"logprobs":null,"finish_reason":"length"}]}

if should return

data:{"id":"","object":"text_completion","created":1710508199,"model":"HuggingFaceH4/zephyr-7b-beta","system_fingerprint":"1.4.3-sha-e6bb3ff","choices":[{"index":0,"delta":{"role":"assistant","content":"I"},"logprobs":null,"finish_reason":null}]}
data:{"id":"","object":"text_completion","created":1710508199,"model":"HuggingFaceH4/zephyr-7b-beta","system_fingerprint":"1.4.3-sha-e6bb3ff","choices":[{"index":0,"delta":{"role":"assistant","content":"'"},"logprobs":null,"finish_reason":null}]}
data:{"id":"","object":"text_completion","created":1710508199,"model":"HuggingFaceH4/zephyr-7b-beta","system_fingerprint":"1.4.3-sha-e6bb3ff","choices":[{"index":0,"delta":{"role":"assistant","content":"m"},"logprobs":null,"finish_reason":"length"}]}

EDIT: I also edited ToolCall.index to be always 0 (instead of the generated token index) but for this one I'm actually unsure. It might be the index of the tool in the array of tools? OpenAI's documentation doesn't provide any information about it:

index integer

---

I also noticed that in OpenAI's example, the last chunk doesn't have a delta and is the only one that has a finish_reason returning. TGI is slightly different since the last chunk has both the last delta (i.e. the last generated token) + the finish reason. I don't think this is worth fixing since it is not a requirement according to the docs/specs (at least not that I know of).

[drbh]

@Wauplin thanks for the fix and references 🙏

LGTM