Bugfixes for CadenceWakeWordQuantizer

Summary: Fixes a bug introducing empty cat operators in the quantizer, and one where the output of a pass is not serializable.

Differential Revision: D95961290

Author: mcremon-meta

backends/cadence/aot/quantizer/fusion_pass.py

@@ -563,6 +563,9 @@ def call(self, graph_module: fx.GraphModule) -> PassResult:  # noqa: C901
                             quant_node,
                         )
                     elif isinstance(pattern, CatPattern):
+                        # Skip fusion if inputs_inputs is empty to avoid creating cat([])
+                        if not inputs_inputs:
+                            continue
                         args, kwargs = get_args_and_kwargs_cat(
                             inputs_inputs, other_inputs, op_node
                         )

backends/cadence/aot/replace_ops.py

@@ -2368,8 +2368,16 @@ def maybe_remove_or_replace(self, node: torch.fx.Node) -> bool:
                 args=([1], full_arg),
                 kwargs={"dtype": full_output_dtype},
             )
-            full_node.meta = node.meta
-            full_node.meta["val"] = [1]
+            full_node.meta = node.meta.copy()
+            # Create a proper FakeTensor for metadata instead of Python list
+            fake_mode = node.meta["val"].fake_mode
+            if fake_mode is not None:
+                with fake_mode:
+                    full_node.meta["val"] = torch.full(
+                        [1], full_arg, dtype=full_output_dtype
+                    )
+            else:
+                full_node.meta["val"] = torch.empty([1], dtype=full_output_dtype, device="meta")
             new_mul_node = node.graph.call_function(
                 torch.ops.aten.mul.Tensor, args=(x_arg, full_node)
             )