Add a symlink for llama_models

[ashwinb]

A much more lightweight attempt to make sure the llama-models package is "correct" -- i.e., it has an expected llama_models sub-directory. There are a few downstream workflows where the repository is merely checked out (without a corresponding pip install -e) and it is expected that from llama_models.<foo> works by just setting a PYTHONPATH.

Test Plan
Produced a package with python -m build and then installed it in a fresh conda environment.

Checked that $CONDA_PREFIX/lib/.../site-packages/llama_models contained expected package structure namely:

datatypes.py  __init__.py  llama3/  llama3_1/  llama3_2/  prompt_format.py  __pycache__/  schema_utils.py  scripts/  sku_list.py

[github-actions]

[raghotham]

Looks good. Thanks.