setBackend() required for ml5.neuralNetwork() bug

The `ml5.neuralNetwork()` examples require either:

```js
ml5.setBackend("webgl");
```

```js
ml5.setBackend("cpu");
```

Without one of the above, the examples break with an error related to `webgpu`, see #34. This came up again while reviewing #105. We'll leave it for now, but it would be great to remove the requirement from the examples and have them all work with `webgl` as default.
