[Feature] Default ignore_eos True for random dataset

[yewentao256]

Purpose

Idea from @mgoin

vllm bench serve --model deepseek-ai/DeepSeek-R1  --dataset-name random --host 127.0.0.1 --port 9256 --random-input-len 4 --random-output-len 1024 --request-rate inf --num-prompts 160 --max-concurrency 32

With overlap

============ Serving Benchmark Result ============
Successful requests:                     160       
Failed requests:                         0         
Maximum request concurrency:             32        
Benchmark duration (s):                  171.55    
Total input tokens:                      480       
Total generated tokens:                  123383    
Request throughput (req/s):              0.93      
Output token throughput (tok/s):         719.24    
Peak output token throughput (tok/s):    832.00    
Peak concurrent requests:                49.00     
Total Token throughput (tok/s):          722.04    
---------------Time to First Token----------------
Mean TTFT (ms):                          168.90    
Median TTFT (ms):                        87.14     
P99 TTFT (ms):                           420.81    
-----Time per Output Token (excl. 1st token)------
Mean TPOT (ms):                          39.90     
Median TPOT (ms):                        40.11     
P99 TPOT (ms):                           40.23     
---------------Inter-token Latency----------------
Mean ITL (ms):                           39.90     
Median ITL (ms):                         39.90     
P99 ITL (ms):                            46.70     
==================================================
Without (main):

============ Serving Benchmark Result ============
Successful requests:                     160       
Failed requests:                         0         
Maximum request concurrency:             32        
Benchmark duration (s):                  166.48    
Total input tokens:                      480       
Total generated tokens:                  124313    
Request throughput (req/s):              0.96      
Output token throughput (tok/s):         746.73    
Peak output token throughput (tok/s):    864.00    
Peak concurrent requests:                50.00     
Total Token throughput (tok/s):          749.62    
---------------Time to First Token----------------
Mean TTFT (ms):                          135.40    
Median TTFT (ms):                        83.12     
P99 TTFT (ms):                           286.72    
-----Time per Output Token (excl. 1st token)------
Mean TPOT (ms):                          38.42     
Median TPOT (ms):                        38.61     
P99 TPOT (ms):                           38.76     
---------------Inter-token Latency----------------
Mean ITL (ms):                           38.43     
Median ITL (ms):                         38.43     
P99 ITL (ms):                            44.55     
==================================================

Total generated tokens is different since ignore_eos is not set, which may be harmful to benchmark.

This PR make ignore_eos defaults to True if random_output_len is set.

Update:
We don't need to check random_output_len since it is already set to 128 by default

[gemini-code-assist]

Code Review

This pull request changes the default behavior of the serve benchmark for random datasets by setting ignore_eos to True. This is a sensible change to ensure that generation proceeds for the full requested length, improving benchmark consistency. My review focuses on improving the clarity and correctness of the implementation. The current logic contains a redundant check and a misleading comment. I've suggested changes to simplify the code and align the comments with the actual behavior, making the code easier to understand and maintain.