[V1] Get input tokens from scheduler

[WoosukKwon]

This PR changes the scheduler and model runner so that the model runner gets the input token IDs from the scheduler. This change is especially useful when the token IDs are not generated by the model runner (e.g., non-last ranks in PP).

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[WoosukKwon]

cc @comaniac This PR seems to work correctly when using TP (or single GPU), but PP still generates gibberish outputs.

[mergify[bot]]

This pull request has merge conflicts that must be resolved before it can be merged. Please rebase the PR, @WoosukKwon.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[WoosukKwon]

@comaniac @njhill @LiuXiaoxuanPKU I've update the PR with some simplification for spec decoding. PTAL.