Tensor lifetimes

[BhavyeMathur]

In previous versions, code like the following would compile without error:

fn undefined_behaviour() {
    let mut a;

    {
        let b = Tensor::zeros([3, 5]);
        a = b.view();
    }

    a.fill(50);
}

Even though a is a view into Tensor b which is destroyed once its scope ends, the library still allowed you to use a. With added lifetime generics for Tensor, code like this now generates a compiler error.