Generation: stop at `eos` for assisted decoding (#31301)
* fix
* move changes to prompt lookup
* add test
* set eos in assistant model
* style
* fix flakiness
* changes for new `main`
* Update tests/generation/test_utils.py
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
* Update tests/generation/test_utils.py
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
* add comment to explain
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Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
