[CI/Build] Match perf warmups to benchmark concurrency

Use enough warmup requests to cover a full concurrent batch while preserving the existing two-request minimum.

Signed-off-by: Sy03 <24841857+Sy0307@users.noreply.github.com>

tests/dfx/perf/scripts/run_benchmark.py

@@ -279,5 +279,6 @@ def test_performance_benchmark(omni_server, benchmark_params):
             random_input_len=params.get("random_input_len"),
             random_output_len=params.get("random_output_len"),
             resource_label=resource_label,
+            num_warmups=max(2, int(concurrency)),
         )
         assert_result(result, params, num_prompt)