When multiple prompts are sent in a single /v1/completions request,
each response needs to carry the correct index so the client can
match results to their corresponding prompts. The index field was
not being set on partial responses, final responses, or embedding
responses, causing batch results to all report index 0.
Set res->index = slot.task->index in send_partial_response,
send_final_response, and send_embedding.
Generated with [Devin](https://cli.devin.ai/docs)
Co-authored-by: Joshua Jolley <jjolley@clearwateranalytics.com>
Co-authored-by: Devin <noreply@cognition.ai>
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