[tinker] Return prompt and top-k prompt logprobs from sample requests

[pcmoritz]

sample requests carrying prompt_logprobs / topk_prompt_logprobs came back with both fields unset, which the Tinker SDK surfaces as "Tinker response did not include prompt_logprobs".

Three gaps, all in the glue between the API layer and the backends:

• api.py accepted topk_prompt_logprobs and dropped it when building types.SampleInput, so it never reached a backend. A positive top-k now also implies prompt_logprobs (both come off the same prompt forward pass).
• The SkyRL-Train backend never put include_prompt_logprobs / topk_prompt_logprobs in the sample body, so vLLM was never asked for them, and then read the result as a list-of-lists when RemoteInferenceClient.sample returns flat per-prompt-token lists -- position 0 (always None) would have been returned as the whole field. Only the first sample of each request asks for prompt logprobs now, since all num_samples samples share one prompt.
• types.SampleOutput.prompt_logprobs was typed list[float], which rejects the None at position 0.

The two forwarding clients in extra/ also gain prompt-logprob support via vLLM's /v1/completions, and the vLLM -> Tinker conversion is now shared (convert_vllm_prompt_logprobs) so the three paths cannot drift.

The JAX backend returns an explicit error for topk_prompt_logprobs: tx's generator only produces the prompt tokens' own logprobs, so there is no per-position distribution to build top-k from.

[gemini-code-assist]

Code Review

This pull request introduces support for requesting top-k prompt logprobs (topk_prompt_logprobs) alongside standard prompt logprobs. It adds a helper function convert_vllm_prompt_logprobs to standardize the conversion of vLLM's prompt logprobs into Tinker's response format. The SkyRL-Train backend is updated to forward and parse these top-k logprobs, while the JAX backend is updated to explicitly reject requests with topk_prompt_logprobs > 0 as it is unsupported. Unit tests have also been added to verify this new functionality. I have no additional feedback to provide.