`pairplot` with 1 dimensional tensor returns an error

**Describe the bug**
After recent changes, `pairplot` can no longer be used to plot the distribution of a 1-dim parameter, and throws `TypeError: 'Axes' object is not subscriptable`

**To Reproduce**
```
import torch
from sbi.analysis import pairplot
theta = torch.randn(1000, 1)
pairplot(theta,limits=[(-1,1)])
```
