I still get a shape mismatch using:
dev = 'cpu'
cnn = ConvNet(recurrent=True).to(dev)
model = Mixnet(
    n_outputs=20,
    num_layers=2,
    hidden_dim=150,
    n_steps=3,
    cnn=cnn).to(dev)

x = torch.randn(16, 4, 3, 257, 490, device=dev)
output = model(x)

> RuntimeError: input.size(-1) must be equal to input_size. Expected 19968, got 2560