Add HasArrayData to GradientProvider

First mentioned in #99.

In order to access parameter data for optimizers, the parameters need to have `HasArrayData` added:

```diff
    fn gradient<P>(&mut self, p: &P) -> Option<Box<P::Array>>
    where
-        P: HasUniqueId + HasArrayType<Dtype = f32> + HasDevice;
+        P: HasUniqueId + HasArrayType<Dtype = f32> + HasDevice + HasArrayData;
```