[Bug Fix] Fix padding when running in NHWC (#9729)

### Summary
There is a bug when there is a constant_pad between two convolutions. In
order to minimize permutes associated with memory format changes, we
sometimes compute ops in NHWC. This is the case for ConstantPad when it
is between two convs:

```
    a = conv(a)
    a = constant_pad(a, paddings=[1, 2, 3, 4])
    a = conv(a)
```

in this case we need to make sure the paddings given to constant_pad are
also permuted to nhwc.

### Test plan
python install_executorch.py --editable

python -m unittest
backends.xnnpack.test.ops.test_static_constant_pad.TestStaticConstantPad.test_fp32_static_constant_pad_nhwc

Author: mcr229

backends/xnnpack/operators/op_static_constant_pad.py

@@ -7,6 +7,7 @@
 from typing import cast, Dict, List
 
 import torch
+
 from executorch.backends.xnnpack.operators.node_visitor import (
     get_tensor_value,
     NodeVisitor,
@@ -17,7 +18,11 @@
     XNNStaticConstantPad,
     XNode,
 )
-from executorch.backends.xnnpack.utils.utils import check_or_raise, get_input_node
+from executorch.backends.xnnpack.utils.utils import (
+    check_or_raise,
+    get_input_node,
+    PERM_NCHW_TO_NHWC,
+)
 
 
 @register_node_visitor
@@ -113,8 +118,15 @@ def define_node(
         # b)
         # tuple[0] = prepadding dim[-1]
         # tuple[1] = postpadding dim[-1]
+        is_channels_last = node.meta.get("XNN_NHWC_NODE", False)
         pre_paddings = all_paddings[-2::-2]  # even index elements in reverse order
         post_paddings = all_paddings[::-2]  # odd index elements in reverse order
+        if is_channels_last:
+            check_or_raise(len(pre_paddings) == 4, "Expecting prepaddings to be 4D")
+            check_or_raise(len(post_paddings) == 4, "Expecting postpaddings to be 4D")
+
+            pre_paddings = [pre_paddings[i] for i in PERM_NCHW_TO_NHWC]
+            post_paddings = [post_paddings[i] for i in PERM_NCHW_TO_NHWC]
 
         # the padding value, which defaults to 0.0
         padding_value = cast(float, node.args[2]) if len(node.args) > 2 else 0.0

backends/xnnpack/test/ops/test_static_constant_pad.py

@@ -14,6 +14,30 @@ class TestStaticConstantPad(unittest.TestCase):
     def setUp(self):
         torch._dynamo.reset()
 
+    class NHWCStaticConstantPad(torch.nn.Module):
+        def __init__(self):
+            super().__init__()
+            self.conv1 = torch.nn.Conv2d(in_channels=2, out_channels=2, kernel_size=1)
+            self.conv2 = torch.nn.Conv2d(in_channels=13, out_channels=13, kernel_size=1)
+
+        def forward(self, x):
+            a = self.conv1(x)
+            pad_6 = (1, 2, 3, 4, 5, 6)
+            a = torch.nn.functional.pad(
+                input=a,
+                pad=pad_6,
+                mode="constant",
+                value=3.1,
+            )
+            # tensorshape = [1, 13, 10, 7]
+            a = self.conv2(a)
+
+            return a
+
+        def sample_inputs(self):
+            # NCHW
+            return (torch.randn(1, 2, 3, 4),)
+
     class StaticConstantPadFunctional(torch.nn.Module):
         def __init__(self):
             super().__init__()
@@ -205,3 +229,24 @@ def test_qs8_static_constant_pad_2d(self):
             .serialize()
             .run_method_and_compare_outputs()
         )
+
+    def test_fp32_static_constant_pad_nhwc(self):
+        conv = self.NHWCStaticConstantPad()
+        inputs = conv.sample_inputs()
+        (
+            Tester(conv, inputs)
+            .export()
+            .check_count({"torch.ops.aten.pad.default": 1})
+            .dump_artifact()
+            .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",
+                    "executorch_exir_dialects_edge__ops_aten_convolution_default",
+                ]
+            )
+            .to_executorch()
+            .serialize()
+            .run_method_and_compare_outputs()
+        )