Vulkan mean errors out during lowering

🐛 Describe the bug

Models with torch.mean error out during lowering.

import torch
from executorch.backends.vulkan.partitioner.vulkan_partitioner import VulkanPartitioner
from executorch.exir import to_edge_transform_and_lower, EdgeCompileConfig, to_edge
from executorch.extension.pybindings.portable_lib import _load_for_executorch_from_buffer
from typing import Callable, List, Optional, Tuple, Union

class Model(torch.nn.Module):
    def __init__(
        self,
    ):
        super().__init__()
        
    def forward(self, x):
        return torch.mean(x, dim=0)
        
model = Model()
inputs = (
    torch.randn(8, 8),
)
eager_outputs = model(*inputs)

ep = torch.export.export(model.eval(), inputs)
print(ep)
lowered = to_edge_transform_and_lower(
    ep,
    partitioner=[VulkanPartitioner()],
    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]

print(f"Inputs: {inputs}")
print(f"Eager: {eager_outputs}")
print(f"ET:    {et_outputs}")

Outputs:

File /data/users/gjcomer/fbsource/buck-out/v2/gen/fbcode/fdcb6705e87e1def/bento_kernels/cria/__bento_kernel_cria_binary__/bento_kernel_cria_binary#link-tree/executorch/backends/vulkan/op_registry.py:457, in register_reduce_op.<locals>.check_reduce_node(node)
    454 if isinstance(dim_list, list) and len(dim_list) != 1:
    455     return False
--> 457 keepdim = node.args[2]
    458 if isinstance(keepdim, bool) and not keepdim:
    459     return False
IndexError: tuple index out of range

Versions

Run on Meta internal master, Jul 3, fbcode/SwiftShader

cc @SS-JIA @manuelcandales @cbilgin

Start with backends/vulkan/op_registry.py, especially register_reduce_op and check_reduce_node, and run the provided torch.mean reproduction with VulkanPartitioner. Trace the exported reduce node arguments during lowering; done means the example lowers and converts to Executorch without the IndexError.