Qualcomm AI Engine Direct - Add test verifying LLM calib/eval limits (#20234)

### Summary
Add `TestExampleLLMScript.test_static_llm_eval_limit`, which compiles a
static
LLM once with a fixed calibration, then evaluates the same pre-generated
pte
twice with different eval limits and asserts the resulting wikitext
perplexity
differs. 

### Test plan
``` bash
python backends/qualcomm/tests/test_qnn_delegate.py   TestExampleLLMScript.test_static_llm_eval_limit  -s  ${SERIAL_NUM} -m SM8750   -b build-android   -a .   --executorch_root .   --model_name smollm2_135m
```

Author: DannyYuyang-quic

backends/qualcomm/tests/test_qnn_delegate.py

@@ -8080,6 +8080,98 @@ def test_static_llm_model(self):  # noqa: C901
                             device_inference_speed, expected_inference_speed
                         )
 
+    def test_static_llm_eval_limit(self):
+        # Verify calib and eval are truly separate: compile once with a fixed
+        # calib, then eval the same pte with different eval limit.
+        if not self.required_envs([self.model_name]):
+            self.skipTest("missing required envs")
+        assert (
+            self.model_name in self.llm_specs
+        ), f"Unable to find {self.model_name} under model_specs."
+
+        def run_llama(extra_cmds):
+            cmds = [
+                "python",
+                f"{self.executorch_root}/examples/qualcomm/oss_scripts/llama/llama.py",
+                "--artifact",
+                self.artifact_dir,
+                "--build_folder",
+                self.build_folder,
+                "--prompt",
+                "I would like to learn python, could you teach me with a simple example?",
+                "--temperature",
+                "0",
+                "--decoder_model",
+                self.model_name,
+                "--model_mode",
+                "kv",
+                "--max_seq_len",
+                "1024",
+                "--max_context_len",
+                "1024",
+                "--skip_user_prompt_calibration",
+                "--soc_model",
+                self.soc_model,
+                "--target",
+                self.target,
+                "--ip",
+                self.ip,
+                "--port",
+                str(self.port),
+                "--seed",
+                str(1126),
+                "--backend",
+                self.backend,
+            ]
+            cmds.extend(extra_cmds)
+            if self.host:
+                cmds.extend(["--host", self.host])
+            elif self.enable_x86_64:
+                cmds.extend(["--enable_x86_64"])
+
+            p = subprocess.Popen(cmds, stdout=subprocess.DEVNULL)
+            with Listener((self.ip, self.port)) as listener:
+                conn = listener.accept()
+                p.communicate()
+                msg = json.loads(conn.recv())
+            if "Error" in msg:
+                self.fail(msg["Error"])
+            return msg
+
+        # Compile once with a fixed calibration so quantization is identical
+        # across the eval runs below.
+        run_llama(
+            [
+                "--compile_only",
+                "--calib_tasks",
+                "wikitext",
+                "--calib_limit",
+                "1",
+            ]
+        )
+
+        def eval_ppl(eval_limit):
+            extra_cmds = [
+                "--pre_gen_pte",
+                self.artifact_dir,
+                "--eval_methods",
+                "tasks_eval",
+                "--eval_tasks",
+                "wikitext",
+                "--eval_limit",
+                str(eval_limit),
+            ]
+            if self.device:
+                extra_cmds.extend(["--device", self.device])
+            return run_llama(extra_cmds)["wiki_ppl"]
+
+        ppl_eval_1 = eval_ppl(1)
+        ppl_eval_3 = eval_ppl(3)
+        logging.info(
+            f"wiki_ppl: eval_limit=1: {ppl_eval_1}, eval_limit=3: {ppl_eval_3}"
+        )
+        self.assertNotEqual(ppl_eval_1, ppl_eval_3)
+
     def test_codegen2_1b(self):
         if not self.required_envs():
             self.skipTest("missing required envs")