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import torch
# from https://github.com/napsternxg/pytorch-practice/blob/master/Pytorch%20-%20MMD%20VAE.ipynb
def compute_kernel(x, y):
x_size = x.size(0)
y_size = y.size(0)
dim = x.size(1)
x = x.unsqueeze(1) # (x_size, 1, dim)
y = y.unsqueeze(0) # (1, y_size, dim)
tiled_x = x.expand(x_size, y_size, dim)
tiled_y = y.expand(x_size, y_size, dim)
kernel_input = (tiled_x - tiled_y).pow(2).mean(2)/float(dim)
return torch.exp(-kernel_input) # (x_size, y_size)
def compute_mmd(x, y):
x_kernel = compute_kernel(x, x)
y_kernel = compute_kernel(y, y)
xy_kernel = compute_kernel(x, y)
mmd = x_kernel.mean() + y_kernel.mean() - 2*xy_kernel.mean()
return mmd
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