Vulkan index select outputs don't match eager/portable

🐛 Describe the bug

When running the following repro on the Vulkan delegate, outputs differ from eager mode. ET portable matches eager.

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

class IndexSelectModel(torch.nn.Module):
    def __init__(self, dim=0):
        super().__init__()
        self.dim = dim
        
    def forward(self, x, indices):
        return torch.index_select(x, self.dim, indices)
        
model = IndexSelectModel(dim=0)
indices = torch.tensor([0, 1], dtype=torch.int64)
inputs = (
    torch.randn(5, 5),
    indices,
)
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:

Inputs: (tensor([[-0.4567, -0.3723, -0.1551, -0.7132, -0.5437],
        [ 1.5363,  0.7774,  1.8513,  1.4016,  0.6614],
        [ 0.2748, -0.6432, -0.8875, -1.6339, -0.4679],
        [-1.1551,  0.0595,  0.4082, -1.3087, -0.7413],
        [-0.1365,  0.1206,  0.6683,  1.3647, -0.3534]]), tensor([0, 1]))
Eager: tensor([[-0.4567, -0.3723, -0.1551, -0.7132, -0.5437],
        [ 1.5363,  0.7774,  1.8513,  1.4016,  0.6614]])
ET:    tensor([[-0.4567, -0.3723, -0.1551, -0.7132, -0.5437],
        [-0.4567, -0.3723, -0.1551, -0.7132, -0.5437]])

Versions

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

cc @SS-JIA @manuelcandales @cbilgin

Start by running the provided IndexSelectModel repro with VulkanPartitioner and compare its output with eager mode and ET portable. Read executorch.backends.vulkan.partitioner.vulkan_partitioner and trace the Vulkan index-select path; done means Vulkan produces the same selected rows as eager and portable execution.