first LLM prototype

[martinjaggi]

we already had the PRs but no issue yet, so adding that for completeness

objective: add a first prototype implementation based on the GPT-tfjs code adaption by @peacefulotter and @s314cy

related
https://github.com/peacefulotter/disco/tree/NAN-llm-s314cy

is addressed partially in those PRs it seems:
#619 and #600