CoreML ReplicationPad3d fails to load at runtime

🐛 Describe the bug

Models with ReplicationPad3d on Core ML fail to load at runtime.

Repro:

import torch

from executorch.backends.apple.coreml.partition import CoreMLPartitioner
from executorch.exir import to_edge_transform_and_lower, EdgeCompileConfig, to_edge
from executorch.extension.pybindings.portable_lib import _load_for_executorch_from_buffer

class Model(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.pad = torch.nn.ReplicationPad3d(padding=2)
        
    def forward(self, x):
        return self.pad(x)

model = Model()
inputs = (
    torch.randn(1, 6, 6, 6, 6),
)

eager_outputs = model(*inputs)
#print(f"Eager: {eager_outputs.shape} {eager_outputs}")

ep = torch.export.export(model.eval(), inputs)
print(ep)

lowered = to_edge_transform_and_lower(
    ep,
    partitioner=[CoreMLPartitioner()],
    compile_config=EdgeCompileConfig(_check_ir_validity=False)
).to_executorch()

print(lowered.exported_program())

et_model = _load_for_executorch_from_buffer(lowered.buffer)
et_outputs = et_model([*inputs])[0]

et_outputs - eager_outputs

Output:

[ETCoreMLModelCompiler.mm:55] [Core ML]  Failed to compile model, error = Error Domain=com.apple.mlassetio Code=1 "Failed to parse the model specification. Error: Unable to parse ML Program: in operation aten_pad_default_cast_fp16: Padding for more than two dimensions only supports `$
[backend_delegate.mm:288] [Core ML] Model init failed Failed to compile model, error = Error Domain=com.apple.mlassetio Code=1 "Failed to parse the model specification. Error: Unable to parse ML Program: in operation aten_pad_default_cast_fp16: Padding for more than two dimension$
[coreml_backend_delegate.mm:193] CoreMLBackend: Failed to init the model.
[method.cpp:113] Init failed for backend CoreMLBackend: 0x23

Versions

coremltools version 8.3
executorch commit https://github.com/pytorch/executorch/commit/67b6009d6b3b67eee775c8ed2fe30eae6e0bb65c (Jun 14)

cc @kimishpatel @YifanShenSZ @cymbalrush @metascroy

Start by running the provided ReplicationPad3d repro through CoreMLPartitioner and inspect the failure at aten_pad_default_cast_fp16. Trace how the exported padding operation is lowered and loaded by the Core ML runtime. Done means the model loads successfully and its output matches eager_outputs.