[V1][WIP] V1 sampler implements parallel sampling (PR 1/N for parallel sampling support)

[afeldman-nm]

The vLLM V1 sampler will support parallel sampling.

Currently the sampler consumes a logits vector which is concatenated over all requests, i.e. the logits vector is total_batch_tokens x vocab_size. Each request contributes a single completion.

Now the sampler will consume a logits vector which is total_tokens x vocab_size where total_tokens is the sum of the sequence lengths of all ongoing completions for all requests, allowing that a request may have more than one completion.

TODO: understand how to exploit prefix caching for parallel sampling?

NOTE: this PR depends on #9880

PR 1/N towards addressing the known need for vLLM v1 parallel sampling support (as described in the vLLM's V1 Engine Architecture RFC)

[github-actions]

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[mergify]

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

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