function hannWindow_(windowLength) {
    return (0, _signalOpsUtil.cosineWindow)(windowLength, 0.5, 0.5);
}
Generate a Hann window.

See: https://en.wikipedia.org/wiki/Window_function#Hann_and_Hamming_windows

```js
tf.signal.hannWindow(10).print();
```
@param The length of window

@doc {heading: 'Operations', subheading: 'Signal', namespace: 'signal'}