fix: don't add eot token if add_eot_token knob is False

[EeyoreLee]

1. add a arg named add_eot_token to avoid to add eot token when args.add_eot_token is False. (fix step2 llama2)
2. remove some redundant code. like chosen_token["input_ids"] = chosen_token["input_ids"]
3. avoid using list as python function args default value.