[Benchmark][Bugfix] Fix SonnetDataset default values in benchmark_throughput.py (#16556)

Author: JenZhao

benchmarks/benchmark_throughput.py

@@ -597,18 +597,26 @@ def validate_args(args):
     parser.add_argument(
         "--prefix-len",
         type=int,
-        default=0,
-        help="Number of fixed prefix tokens before the random "
-        "context in a request (default: 0).",
-    )
+        default=None,
+        help=f"Number of prefix tokens to be used in RandomDataset "
+        "and SonnetDataset. For RandomDataset, the total input "
+        "length is the sum of prefix-len (default: "
+        f"{RandomDataset.DEFAULT_PREFIX_LEN}) and a random context length "
+        "sampled from [input_len * (1 - range_ratio), "
+        "input_len * (1 + range_ratio)]. For SonnetDataset, "
+        f"prefix_len (default: {SonnetDataset.DEFAULT_PREFIX_LEN}) "
+        "controls how much of the input is fixed lines versus "
+        "random lines, but the total input length remains approximately "
+        "input_len tokens.")
     # random dataset
     parser.add_argument(
         "--random-range-ratio",
         type=float,
-        default=0.0,
-        help="Range ratio for sampling input/output length, "
-        "used only for RandomDataset. Must be in the range [0, 1) to define "
-        "a symmetric sampling range "
+        default=None,
+        help=f"Range ratio (default : {RandomDataset.DEFAULT_RANGE_RATIO}) "
+        "for sampling input/output length, "
+        "used only for RandomDataset. Must be in the range [0, 1) to "
+        "define a symmetric sampling range "
         "[length * (1 - range_ratio), length * (1 + range_ratio)].",
     )