Thanks for the rest of the code.
While running it I get a proper error message:
RuntimeError: Tensor for argument #2 'mat1' is on CPU, but expected it to be on GPU (while checking arguments for addmm)

which points to a device mismatch in Generator.
After checking the code it seems that z is created on the CPU, while the parameters of the model are on the GPU.
Use
z = self.latent.sample([n, self.n_filters]).cuda()

or
z = self.latent.sample([n, self.n_filters]).to(next(self.parameters()).device)

to fix this issue.