[Sampler] Support returning final logprobs (#22387)

Signed-off-by: 22quinn <33176974+22quinn@users.noreply.github.com>
Co-authored-by: Nick Hill <nhill@redhat.com>
Co-authored-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

vllm/engine/arg_utils.py

@@ -516,6 +516,7 @@ class EngineArgs:
         model_group.add_argument("--max-logprobs",
                                  **model_kwargs["max_logprobs"])
         model_group.add_argument("--logprobs-mode",
+                                 choices=[f.value for f in LogprobsMode],
                                  **model_kwargs["logprobs_mode"])
         model_group.add_argument("--disable-sliding-window",
                                  **model_kwargs["disable_sliding_window"])