GPT4All API: Add Streaming to the Completions endpoint

[TheDropZone]

Describe your changes

Adding the ability to do streaming requests to the "/completions" endpoint of the API. Added in a test to showcase using the openai python client to stream the response from the api.

This addresses the NotImplementedError in the completions route of the api
https://github.com/nomic-ai/gpt4all/blob/main/gpt4all-api/gpt4all_api/app/api_v1/routes/completions.py#L54

Issue ticket number and link

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• I have added thorough documentation for my code.
• I have tagged PR with relevant project labels. I acknowledge that a PR without labels may be dismissed.
• If this PR addresses a bug, I have provided both a screenshot/video of the original bug and the working solution.

Demo

    model = "gpt4all-j-v1.3-groovy"
    prompt = "Who is Michael Jordan?"
    for resp in openai.Completion.create(
            model=model,
            prompt=prompt,
            max_tokens=50,
            temperature=0.28,
            top_p=0.95,
            n=1,
            echo=True,
            stream=True):
        print(resp.choices[0].text)

The above results in the model output being streamed to your console, directly from the API

Notes

There is a great issue thread over on the openai-node library page, discussing a lot of this streaming work.
openai/openai-node#18

[AndriyMulyar]

Great contribution. Looks good from first glance. I will review on Wednesday after the holiday!

[AndriyMulyar]

Unit tests pass on my local build. Merging.