Adding CLI arg to select model type for ET runner (pytorch#935)

Author: Jack-Khuu

README.md

@@ -284,7 +284,7 @@ scripts/build_native.sh et
 
 Execute using the runner
 ```bash
-cmake-out/et_run llama3.pte -z `python3 torchchat.py where llama3`/tokenizer.model -i "Once upon a time"
+cmake-out/et_run llama3.pte -z `python3 torchchat.py where llama3`/tokenizer.model -l 3 -i "Once upon a time"
 ```
 
 </details>