test: support return logprobs in bench_offline_throughput test (#12462)

Signed-off-by: Zhao Chen <zhaochen.zju@gmail.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

python/sglang/bench_offline_throughput.py

@@ -60,6 +60,8 @@ class BenchArgs:
     skip_warmup: bool = False
     do_not_exit: bool = False
     prompt_suffix: str = ""
+    return_logprob: bool = False
+    logprob_start_len: int = -1
 
     @staticmethod
     def add_cli_args(parser: argparse.ArgumentParser):
@@ -187,6 +189,17 @@ class BenchArgs:
             default="",
             help="Suffix applied to the end of all user prompts, followed by assistant prompt suffix.",
         )
+        parser.add_argument(
+            "--return-logprob",
+            action="store_true",
+            help="Enable returning log probabilities.",
+        )
+        parser.add_argument(
+            "--logprob-start-len",
+            type=int,
+            default=-1,
+            help="Start length for logprob. -1 means only return logprobs for output tokens (default). 0 means return logprobs for all tokens including input.",
+        )
 
     @classmethod
     def from_cli_args(cls, args: argparse.Namespace):
@@ -201,6 +214,8 @@ def throughput_test_once(
     ignore_eos: bool,
     extra_request_body: Dict,
     profile: bool,
+    return_logprob: bool = False,
+    logprob_start_len: int = -1,
 ):
     measurement_results = {
         "backend": backend_name,
@@ -233,7 +248,12 @@ def throughput_test_once(
         backend.start_profile()
 
     st = time.perf_counter()
-    gen_out = backend.generate(prompt=prompt, sampling_params=sampling_params)
+    gen_out = backend.generate(
+        prompt=prompt,
+        sampling_params=sampling_params,
+        return_logprob=return_logprob,
+        logprob_start_len=logprob_start_len,
+    )
     latency = time.perf_counter() - st
 
     if profile:
@@ -355,6 +375,8 @@ def throughput_test(
             ignore_eos=not bench_args.disable_ignore_eos,
             extra_request_body=extra_request_body,
             profile=False,
+            return_logprob=bench_args.return_logprob,
+            logprob_start_len=bench_args.logprob_start_len,
         )
         time.sleep(0.5)
 
@@ -366,6 +388,8 @@ def throughput_test(
         ignore_eos=not bench_args.disable_ignore_eos,
         extra_request_body=extra_request_body,
         profile=bench_args.profile,
+        return_logprob=bench_args.return_logprob,
+        logprob_start_len=bench_args.logprob_start_len,
     )
     backend.shutdown()