SpecDec Bench: PostProcess flag (#759)

## What does this PR do?

**Type of change:** ? Bug Fix: https://nvbugspro.nvidia.com/bug/5795144

**Overview:** ? Pass postprocess flag to handle slicing message.

## Usage
<!-- You can potentially add a usage example below. -->

```python
# Add a code snippet demonstrating how to use this
```

## Testing
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- **Did you write any new necessary tests?**: Yes/No
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## Additional Information
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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

## Release Notes

* **New Features**
* Introduced --postprocess command-line option to select postprocessing
strategy. Users can choose "base" (default, preserves existing behavior)
or "gptoss" (new alternative method) with validation to reject invalid
selections.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Izzy Putterman <iputterman@nvidia.com>

examples/specdec_bench/README.md

@@ -28,7 +28,7 @@ MTBench is available [here](https://huggingface.co/datasets/HuggingFaceH4/mt_ben
 Download `nvidia/gpt-oss-120b-Eagle3` to a local directory `/path/to/eagle`.
 
 ```bash
-python3 run.py --model_dir openai/gpt-oss-120b --tokenizer openai/gpt-oss-120b --draft_model_dir /path/to/eagle --mtbench question.jsonl --tp_size 1 --ep_size 1 --draft_length 3 --output_length 4096 --num_requests 80 --engine TRTLLM --concurrency 1
+python3 run.py --model_dir openai/gpt-oss-120b --tokenizer openai/gpt-oss-120b --draft_model_dir /path/to/eagle --mtbench question.jsonl --tp_size 1 --ep_size 1 --draft_length 3 --output_length 4096 --num_requests 80 --engine TRTLLM --concurrency 1 --postprocess gptoss
 
 ```
 

examples/specdec_bench/run.py

@@ -18,7 +18,13 @@ import asyncio
 
 import yaml
 from specdec_bench import datasets, metrics, models, runners
-from specdec_bench.utils import decode_chat, encode_chat, get_tokenizer, postprocess_base
+from specdec_bench.utils import (
+    decode_chat,
+    encode_chat,
+    get_tokenizer,
+    postprocess_base,
+    postprocess_gptoss,
+)
 
 engines_available = {
     "TRTLLM": models.TRTLLMPYTModel,
@@ -109,7 +115,12 @@ def run_simple(args):
         metrics_list.insert(0, metrics.AcceptanceRate())
     runner = runners.SimpleRunner(model, metrics=metrics_list)
 
-    postprocess = postprocess_base
+    if args.postprocess == "base":
+        postprocess = postprocess_base
+    elif args.postprocess == "gptoss":
+        postprocess = postprocess_gptoss
+    else:
+        raise ValueError(f"Invalid postprocess: {args.postprocess}")
 
     asyncio.run(
         run_loop(runner, dataset, tokenizer, args.output_length, postprocess, args.concurrency)
@@ -183,6 +194,15 @@ if __name__ == "__main__":
         help="Maximum number of concurrent requests",
     )
     parser.add_argument("--aa_timing", action="store_true", help="Enable AA timing metric")
+    parser.add_argument(
+        "--postprocess",
+        type=str,
+        required=False,
+        default="base",
+        choices=["base", "gptoss"],
+        help="Postprocess to use",
+    )
+
     args = parser.parse_args()
 
     if args.runtime_params is not None: