[onnx] Fix onnx.Shape lowering with scalar input

[jinchen62]

Address nod-ai/SHARK-ModelDev#826

[zjgarvey]

I get a segfault with this when running

--convert-torch-onnx-to-torch --convert-torch-to-linalg

on

module {
  func.func @main(%arg0: !torch.vtensor<[],f32>) -> !torch.vtensor<[?],si64> attributes {torch.onnx_meta.ir_version = 10 : si64, torch.onnx_meta.opset_version = 21 : si64, torch.onnx_meta.producer_name = "", torch.onnx_meta.producer_version = ""} {
    %none = torch.constant.none
    %0 = torch.operator "onnx.Shape"(%arg0) : (!torch.vtensor<[],f32>) -> !torch.vtensor<[?],si64> 
    return %0 : !torch.vtensor<[?],si64>
  }
}

As a side note, I ran an onnx model with this setup in onnxruntime, and the output was literally an empty numpy array...

array([], dtype=int64)

[jinchen62]

@zjgarvey I tested this case and it got lowered. Let me double check.

[zjgarvey]

LGTM