[TorchONNXToTorch] Failed to lower torch.operator "onnx.Einsum"

Nobody has claimed this yet.

torch-mlir fails with the following error when running the torch-onnx-to-torch-backend-pipeline on the test case below:

`einsum.mlir:3:10: error: 'torch.aten.permute' op expected input and output tensors to have same rank, but 6 != 25.
%0 = torch.operator "onnx.Einsum"(%arg0, %arg1) {torch.onnx.equation = "...bt pide , ...bo pie -> ...bot pid"} : (!torch.vtensor<[1,1,2,2,3,3],f32>, !torch.vtensor<[1,280,2,2,3],f32>) -> !torch.vtensor<[1,280,1,2,2,3],f32>
^
einsum.mlir:3:10: note: see current operation: %84 = "torch.aten.permute"(%arg0, %83) : (!torch.vtensor<[1,1,2,2,3,3],f32>, !torch.list) -> !torch.vtensor<[1,1,2,2,3,3,94256647605752,94256647577264,6,1,94256647577104,0,1,280,2,2,3,94256647605752,94256647577360,5,94256647577104,0,1,1,280],f32>`

The error can be reproduces using the following command `torch-mlir-opt --torch-onnx-to-torch-backend-pipeline einsum.mlir` with the following test case:

[einsum.mlir](https://github.com/user-attachments/files/26718356/einsum.txt)

Start by running torch-mlir-opt --torch-onnx-to-torch-backend-pipeline einsum.mlir using the attached einsum.mlir test case. Trace the lowering of torch.operator "onnx.Einsum" and the generated torch.aten.permute operation, then ensure the pipeline handles this case without the rank mismatch error.

Written by the indexing model from the issue text.