If param is not used in loss calculation, `Optimizer::update()` causes runtime panic

Contrived example:

```rust
fn main() {
    let mut model: (Linear<5, 5>, Linear<5, 4>) = Default::default();
    let y = model.1.forward(Tensor1D::zeros().traced());
    let gradients = y.square().mean().backward();
    let mut opt: Sgd = Default::default();
    opt.update(&mut model, gradients);
}
```