AvgPool2d in Downsample

[JJZHK]

In Downsample, I think it should be torch.nn.AvgPool2d(kernel_size=(3, 3), stride=(2, 2), padding=(1, 1)), not torch.nn.AvgPool2d(kernel_size=(2,2)). otherwise x & y have wrong size, and x + y will throw exception.

[Pritam-N]

@JJZHK Based on the architecture in the paper, it has avgpool (2,2) and the code is working.