[core] overhaul memory profiling and fix backward compatibility (#10511)

Signed-off-by: youkaichao <youkaichao@gmail.com>

tests/entrypoints/llm/test_gpu_utilization.py

@@ -0,0 +1,25 @@
+from vllm import LLM, SamplingParams
+
+
+def test_gpu_memory_utilization():
+    prompts = [
+        "Hello, my name is",
+        "The president of the United States is",
+        "The capital of France is",
+        "The future of AI is",
+    ]
+    sampling_params = SamplingParams(temperature=0.8, top_p=0.95)
+
+    # makes sure gpu_memory_utilization is per-instance limit,
+    # not a global limit
+    llms = [
+        LLM(model="facebook/opt-125m",
+            gpu_memory_utilization=0.3,
+            enforce_eager=True) for i in range(3)
+    ]
+    for llm in llms:
+        outputs = llm.generate(prompts, sampling_params)
+        for output in outputs:
+            prompt = output.prompt
+            generated_text = output.outputs[0].text
+            print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")