Generate: speculative decoding (#27979)
* speculative decoding
* fix test
* space
* better comments
* remove redundant test
* test nit
* Apply suggestions from code review
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
* PR comments
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Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
