Fix UnboundLocalError: local variable 'qnn_compile_spec_buffer' refer… (#15881)

As title, addressed issues exposed from
pytorch/pytorch#168098
```
FAILED exir/backend/test/test_debug_handle_map.py::TestBackendDebugHandle::test_lowered_the_whole_model - UnboundLocalError: local variable 'qnn_compile_spec_buffer' referenced before assignment
Falsifying example: test_lowered_the_whole_model(
    unlift=False,
    self=<test_debug_handle_map.TestBackendDebugHandle testMethod=test_lowered_the_whole_model>,
)

```

Author: cccclai

backends/qualcomm/partition/utils.py

@@ -16,11 +16,19 @@
 def generate_qnn_executorch_option(
     compiler_specs: List[CompileSpec],
 ) -> bytes:
+    qnn_compile_spec_buffer = None
+
     for compiler_spec in compiler_specs:
         if compiler_spec.key == QCOM_QNN_COMPILE_SPEC:
             qnn_compile_spec_buffer = compiler_spec.value
         else:
             raise ValueError(f"unknown compiler spec key value: {compiler_spec.key}")
+
+    if qnn_compile_spec_buffer is None:
+        raise ValueError(
+            f"QNN compile spec (key={QCOM_QNN_COMPILE_SPEC}) not found in compiler_specs"
+        )
+
     return qnn_compile_spec_buffer