Migrate to tokenizer + fastchat conv templates

[natolambert]

Realistically, doing our own is a losing battle, let's migrate to something maintained.

[hamishivi]

There is a start at this here: https://github.com/allenai/open-instruct/blob/main/eval/templates.py#L112
But more formal integrations with fastchat would be good. I think supporting more flexible generation in general is good. E.g. llama 3 with its multiple eos tokens.

[yizhongw]

I came aross this issue when I want to use a different chat template to train models. So, I made this PR. It should support user-defined chat templates.