Symmetric Metrics are not in [0, 1]

MWE:

```python
In [1]: import exponax as ex

In [2]: import jax

In [3]: a = jax.random.normal(jax.random.key(0), (2, 1, 100))

In [4]: ex.metrics.sRMSE(a[0], a[1])
Out[4]: Array(1.5277674, dtype=float32)
```