thanks very much @ptrblck. That worked but I got another error:
RuntimeError                              Traceback (most recent call last)
<ipython-input-5-2439e6bd56c4> in <module>()
     34 
     35 
---> 36         y = unet_resnet(imgs)
     37         loss = cross_entropy_loss(y, masks)
     38 

1 frames
/usr/local/lib/python3.6/dist-packages/torch/nn/modules/module.py in _call_impl(self, *input, **kwargs)
    720             result = self._slow_forward(*input, **kwargs)
    721         else:
--> 722             result = self.forward(*input, **kwargs)
    723         for hook in itertools.chain(
    724                 _global_forward_hooks.values(),

/content/drive/My Drive/Colab Notebooks/newExtract/network.py in forward(self, x)
     67         dec5 = self.dec5(torch.cat([center, conv5], 1))
     68         dec4 = self.dec4(torch.cat([dec5, conv4], 1))
---> 69         dec3 = self.dec3(torch.cat([dec4, conv3], 1))
     70         dec2 = self.dec2(torch.cat([dec3, conv2], 1))
     71         dec1 = self.dec1(dec2)

RuntimeError: Sizes of tensors must match except in dimension 2. Got 63 and 64 (The offending index is 0)

please How can I make them both 64?