I think you are missing the initialization of the nn.Softplus module and might be running into this error:
torch.randn(1) + nn.Softplus(torch.tensor(1.))
> TypeError: unsupported operand type(s) for +: 'Tensor' and 'Softplus'

You would have to create the module first before applying it onto the tensor or use the functional API:
torch.randn(1) + nn.Softplus()(torch.tensor(1.))
torch.randn(1) + F.softplus(torch.tensor(1.))