[V1][Spec decode] Move drafter to model runner

[WoosukKwon]

This PR moves the speculative decoding drafter, from the engine core to the model runner. At the end of each step, taking the large model’s outputs or hidden states, the spec decoding drafter generates extra "unverified" tokens. Those extra tokens are scheduled and verified in the next step, and in this same step, the spec decoding method generates new extra tokens again.

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[WoosukKwon]

cc @LiuXiaoxuanPKU

[LiuXiaoxuanPKU]

LGTM! Thanks for moving the propose logic to the end of model runner!