float division by zero in benchmark

Command:

` python benchmarks/benchmark_serving.py --tokenizer /home//data/tokenizer.model --num-prompts 300 --dataset-path /home//data/ShareGPT_V3_unfiltered_cleaned_split.json --dataset sharegpt --save-request-outputs`

Logs:

> File "/home//JetStream/benchmarks/benchmark_serving.py", line 778, in
> main(parsed_args)
> File "/home//JetStream/benchmarks/benchmark_serving.py", line 574, in main
> benchmark_result, request_outputs = asyncio.run(
> File "/usr/lib/python3.10/asyncio/runners.py", line 44, in run
> return loop.run_until_complete(main)
> File "/usr/lib/python3.10/asyncio/base_events.py", line 649, in run_until_complete
> return future.result()
> File "/home//JetStream/benchmarks/benchmark_serving.py", line 453, in benchmark
> metrics = calculate_metrics(
> File "/home//JetStream/benchmarks/benchmark_serving.py", line 343, in calculate_metrics
> per_token_latencies.append(outputs[i].latency / output_len)
> ZeroDivisionError: float division by zero