Fix default value of cat_dim. (#11403)

Summary:

This diff fixes the bug introduced by the D75467583, which broke the sharding tests, particulary test_dsp_and_tce_quantized_deepspeech.

Reviewed By: hsharma35

Differential Revision: D76057466

Author: eigen-k

backends/cadence/aot/memory_constraints.py

@@ -360,7 +360,7 @@ def is_cat_along_outermost_dim(
         if len(cat_node.args) > 1:
             cat_dim = cat_node.args[1]
         else:
-            cat_dim = cat_node.kwargs.get("dim", None)
+            cat_dim = cat_node.kwargs.get("dim", 0)
         if not isinstance(cat_dim, int):
             raise ValueError("cat_dim must be an integer.")