Examples:
  >>> # pool of square window of size=3, stride=2
  >>> m = nn.AvgPool3d(3, stride=2)
  >>> # pool of non-square window
  >>> m = nn.AvgPool3d((3, 2, 2), stride=(2, 1, 2))
  >>> input = torch.randn(20, 16, 50, 44, 31)
  >>> output = m(input)
