Make finish_reason nullable in CreateCompletionResponse

[davidmigloz]

Currently, the completion object is used for both streamed and non-streamed modes. However, there's a subtle difference between the two modes: when using streaming finish_reason is null until the last emission.

Example:

curl https://api.openai.com/v1/completions \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer {api_key}" \
  -d '{
    "model": "gpt-3.5-turbo-instruct",
    "prompt": "Say this is a test",
    "temperature": 0,
    "stream": true
  }'

data: {"id":"cmpl-8DRxiREy3SiZNrgxq9DScOdNBxwRJ","object":"text_completion","created":1698216442,"choices":[{"text":"\n\n","index":0,"logprobs":null,"finish_reason":null}],"model":"gpt-3.5-turbo-instruct"}

data: {"id":"cmpl-8DRxiREy3SiZNrgxq9DScOdNBxwRJ","object":"text_completion","created":1698216442,"choices":[{"text":"This","index":0,"logprobs":null,"finish_reason":null}],"model":"gpt-3.5-turbo-instruct"}

data: {"id":"cmpl-8DRxiREy3SiZNrgxq9DScOdNBxwRJ","object":"text_completion","created":1698216442,"choices":[{"text":" is","index":0,"logprobs":null,"finish_reason":null}],"model":"gpt-3.5-turbo-instruct"}

data: {"id":"cmpl-8DRxiREy3SiZNrgxq9DScOdNBxwRJ","object":"text_completion","created":1698216442,"choices":[{"text":" a","index":0,"logprobs":null,"finish_reason":null}],"model":"gpt-3.5-turbo-instruct"}

data: {"id":"cmpl-8DRxiREy3SiZNrgxq9DScOdNBxwRJ","object":"text_completion","created":1698216442,"choices":[{"text":" test","index":0,"logprobs":null,"finish_reason":null}],"model":"gpt-3.5-turbo-instruct"}

data: {"id":"cmpl-8DRxiREy3SiZNrgxq9DScOdNBxwRJ","object":"text_completion","created":1698216442,"choices":[{"text":".","index":0,"logprobs":null,"finish_reason":null}],"model":"gpt-3.5-turbo-instruct"}

data: {"id":"cmpl-8DRxiREy3SiZNrgxq9DScOdNBxwRJ","object":"text_completion","created":1698216442,"choices":[{"text":"","index":0,"logprobs":null,"finish_reason":"stop"}],"model":"gpt-3.5-turbo-instruct"}

data: [DONE]

The current spec does not allow the finish_reason field to be null, which creates issues when using streaming in a client generated from the spec.

Assuming that keeping the same schema for both modes is the preferred option, this PR makes the finish_reason field nullable.

An alternative would be to split the CreateCompletionResponse schema into CreateCompletionResponse and CreateCompletionStreamResponse like it is done in the ChatCompletionResponse definition.