[Bugfix] handle prompt_logprobs in _apply_min_tokens_penalty

[tjohnson31415]

I implemented _apply_min_tokens_penalty in #3124, but was not aware that having prompt_logprobs in the sampling parameters affects the shape of logits. This PR fixes a bug that would occur if using min_tokens and prompt_logprobs together where the start_idx cursor would not be tracking across sequences correctly. The fix is to detect sequences with prompt_logprobs and skip over the rows in logits for all but the last token in the prompt.

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[njhill]

Thanks @tjohnson31415!

Do you think you could add a test? Could just set stop_token_ids to be the id of the first token expected?

[tjohnson31415]

@njhill I have added some tests with prompt_logprobs: I added a few more test cases to the test_sampler_min_tokens_penalty tests and updated the request generation / validation accordingly.

I also added a few sentences to the Sampler doc string to describe the situation with prompt_logprobs adding rows to logits. Let me know if that is clear.

[njhill]

@tjohnson31415 this looks great thank you! Sorry, I didn't realize the test changes would be so involved.

This changes the tests to assume that each batch will comprise only either all prompt or all decode seq groups right? Which will always be the case right now but I'm concerned might not be soon with the chunked prefill changes. So we may need to change the test(s) again at that point to cover hybrid batches.

FYI @rkooo567 ... with hybrid batches, will ordering be such that all prefill sequences will come before the decode sequences?

[rkooo567]

@njhill yeah ordered by prefill -> decode (within input_tokens). The ordering can be controlled from the scheduler.