u-net_brain_mri_vaiq_int8 model support

[AmosLewis]

Failed op in u-net_brain_mri_vaiq_int8.default.onnx.torch.elide.mlir

• torch.aten.convolution to linalg

(mlir_venv) ➜  u-net_brain_mri_vaiq_int8 git:(reduceprod) ✗ torch-mlir-opt --torch-backend-to-linalg-on-tensors-backend-pipeline u-net_brain_mri_vaiq_int8.default.onnx.torch.mlir 
u-net_brain_mri_vaiq_int8.default.onnx.torch.mlir:238:12: error: failed to legalize operation 'torch.aten.convolution' that was explicitly marked illegal
    %221 = torch.aten.convolution %212, %216, %220, %80, %59, %58, %true, %59, %int1 : !torch.vtensor<[32,512,16,16],f32>, !torch.vtensor<[512,256,2,2],f32>, !torch.vtensor<[256],f32>, !torch.list<int>, !torch.list<int>, !torch.list<int>, !torch.bool, !torch.list<int>, !torch.int -> !torch.vtensor<[32,256,32,32],f32>