Utils/visualise can create an IndexError

There is a potential issue in `utils/visualise` at line 43 where `test_slice` is defined:

```python
test_slice = random.randint(0, val_data["image"].shape[4])
```

This will create an `IndexError` if `test_slice == val_data["image"].shape[4]`.

The fix is to adjust `randint`:

```python
test_slice = random.randint(0, val_data["image"].shape[4]-1)
```