[V1][Spec Decode] Change Spec Decode Rejection Sampling API

[LiuXiaoxuanPKU]

This PR tries to change the API of rejection sampler of speculative decoding. This is the preliminary step to support random sampling.

Tasks outside the scope of this PR.

1. write draft_token_ids to persistent batch, which can avoid list operations to convert tensor.
2. implement random sampling here and here. Rejection sampling guarantees that the output distribution with spec decode matches that without spec decode. The drafting method/distribution will not affect the correctness, but only affect the token acceptance rate.

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[WoosukKwon]

LGTM!