Accept default padding value for torch.constant_pad_nd (#7469)

Summary:

xnnpack delegation for pad op assumes that the pad value is always present. However, constant_pad_nd defults to padding value of 0.0 if it's not present in the op. When absent, it errors out in `padding_value = cast(float, node.args[2])` with `IndexError: tuple index out of range`.

{F1974161274}

This diff defaults to padding value of 0.0 if the arg is absent from torch.constant_pad_nd op.

Reviewed By: tarun292

Differential Revision: D67756862

backends/xnnpack/operators/op_static_constant_pad.py

@@ -116,11 +116,14 @@ def define_node(
         pre_paddings = all_paddings[-2::-2]  # even index elements in reverse order
         post_paddings = all_paddings[::-2]  # odd index elements in reverse order
 
+        # the padding value, which defaults to 0.0
+        padding_value = cast(float, node.args[2]) if len(node.args) > 2 else 0.0
+
         ser_node = XNode(
             xnode_union=XNNStaticConstantPad(
                 pre_paddings=pre_paddings,
                 post_paddings=post_paddings,
-                padding_value=cast(float, node.args[2]),
+                padding_value=padding_value,
                 input_id=input_id,
                 output_id=output_id,
                 flags=0,

backends/xnnpack/test/ops/test_static_constant_pad.py

@@ -114,6 +114,40 @@ def test_fp32_static_constant_pad_functional(self):
         )
         self._test_static_constant_pad_functional(inputs)
 
+    def test_constant_pad_nd(self):
+        class ConstantPad(torch.nn.Module):
+            def __init__(self):
+                super().__init__()
+
+            def forward(self, x, y, z):
+                pad_6 = (1, 2, 3, 4, 5, 6)
+                pad_4 = (1, 2, 3, 4)
+                pad_2 = (1, 2)
+                a = torch.constant_pad_nd(input=x, pad=pad_6)
+                b = torch.constant_pad_nd(input=y, pad=pad_4)
+                c = torch.constant_pad_nd(input=z, pad=pad_2)
+
+                return (a + a, b + b, c + c)
+
+        inputs = (
+            torch.randn(size=(5, 4, 3, 2)),
+            torch.randn(size=(5, 3, 2)),
+            torch.randn(size=(4, 3)),
+        )
+        (
+            Tester(ConstantPad(), inputs)
+            .export()
+            .check_count({"torch.ops.aten.constant_pad_nd.default": 3})
+            .to_edge_transform_and_lower()
+            .check_count({"torch.ops.higher_order.executorch_call_delegate": 1})
+            .check_not(
+                ["executorch_exir_dialects_edge__ops_aten_constant_pad_nd_default"]
+            )
+            .to_executorch()
+            .serialize()
+            .run_method_and_compare_outputs()
+        )
+
     def test_qs8_static_constant_pad_functional(self):
         class Pad(torch.nn.Module):
             def __init__(self):