[Mypy] Better fixes for the mypy issues in vllm/config (#37902)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

benchmarks/benchmark_prefix_caching.py

@@ -32,7 +32,6 @@ import dataclasses
 import json
 import random
 import time
-from dataclasses import fields
 
 from transformers import PreTrainedTokenizerBase
 
@@ -197,7 +196,7 @@ def main(args):
 
     engine_args = EngineArgs.from_cli_args(args)
 
-    llm = LLM(**{f.name: getattr(engine_args, f.name) for f in fields(engine_args)})
+    llm = LLM.from_engine_args(engine_args)
 
     sampling_params = SamplingParams(
         temperature=0,