Create top-level torchchat.py CLI binary

[mergennachin]

python torchchat.py {generate, export, eval} --foo --bar

but also you can do:

python export.py --foo --bar

python generate.py --foo --bar

python eval.py --foo --bar

Test Plan:

Added CI test

 mkdir -p checkpoints/stories15M
 cd checkpoints/stories15M
 curl -fsSL -O https://huggingface.co/karpathy/tinyllamas/resolve/main/stories15M.pt
 curl -fsSL -O https://github.com/karpathy/llama2.c/raw/master/tokenizer.model
 cd ../../
 
 export MODEL_PATH=checkpoints/stories15M/stories15M.pt
 export MODEL_NAME=stories15M
 export MODEL_DIR=/tmp

 python generate.py --device cpu --checkpoint-path ${MODEL_PATH} --temperature 0
 python torchchat.py generate --device cpu --checkpoint-path ${MODEL_PATH} --temperature 0