Error occured when convert pytorch model including concatenate op to relay

I am stumbled by an error when I converted an pytorch model containing concatenate op to relay. I draw the concerned part from the model code and form a simple test script as below.

import torch
import torch.nn as nn
import torch.nn.functional as F
import os
os.environ["path"] = os.getenv("path") + ";" + "C:/Program Files/LLVM/bin"
import sys
sys.path.append("D:/workspace/tvm/python")
import tvm

class Up(nn.Module):
    """Upscaling then double conv"""

    def __init__(self):
        super().__init__()

    def forward(self, x1, x2):
        # input is CHW
        diffY = x2.size()[2] - x1.size()[2]
        diffX = x2.size()[3] - x1.size()[3]

        x1 = F.pad(x1, [diffX // 2, diffX - diffX // 2,
                        diffY // 2, diffY - diffY // 2])

        x = torch.cat([x2, x1], dim=1)
        return x

if __name__ == "__main__":
    x1 = torch.randn(1, 32, 12, 14)
    x2 = torch.randn(1, 32, 13, 14)
    up = Up()
    r = up(x1, x2)
    traced_model = torch.jit.trace(up, (x1, x2))
    shape_list = [(i.debugName().split('.')[0],
                   (i.type().sizes(), str(i.type().dtype()).split('.')[1])) for i in
                  list(traced_model.graph.inputs())[1:]]
    mod, params = tvm.relay.frontend.pytorch.from_pytorch(traced_model, shape_list)