[CI/Build] Fix `test_defaults_with_usage_context` in AMD CI (#27926)

Signed-off-by: zhewenli <zhewenli@meta.com>

tests/v1/engine/test_engine_args.py

@@ -53,10 +53,12 @@ def test_defaults_with_usage_context():
     vllm_config: VllmConfig = engine_args.create_engine_config(UsageContext.LLM_CLASS)
 
     from vllm.platforms import current_platform
+    from vllm.utils.mem_constants import GiB_bytes
 
+    device_memory = current_platform.get_device_total_memory()
     device_name = current_platform.get_device_name().lower()
-    if "h100" in device_name or "h200" in device_name:
-        # For H100 and H200, we use larger default values.
+    if device_memory >= 70 * GiB_bytes and "a100" not in device_name:
+        # For GPUs like H100, H200, and MI300x with >= 70GB memory
         default_llm_tokens = 16384
         default_server_tokens = 8192
         default_max_num_seqs = 1024