Add an auto-download script that supports 7B, 13B & 30B models

[nsarrazin]

This PR ads an autodownload script that fetches the model weights from huggingface. This simplifies the initial setup of Serge to just :

git clone git@github.com:nsarrazin/serge.git && cd serge

cp .env.sample .env

docker compose up -d
docker compose exec api python3 /usr/src/app/utils/download.py tokenizer 7B

This also adds support for the 30B theoretically, although I haven't tried it.

This PR closes #2, #13