Push torchtune install deeper in generate._gen_model_input and undo import order change for etmodel

Follow up patch from https://github.com/pytorch/torchchat/pull/1539 which was meant to make torchtune an optional import --- Tested with: Success only when torchtune installed ``` python torchchat.py generate llama3.2-11B --prompt "What's in this image?" --image-prompt assets/dog.jpg --num-samples 3 ``` Success regardless of torchtune install ``` python3 torchchat.py generate llama3.1-base --prompt "Once upon a time," ```