pairplot with 1 dimensional tensor returns an error

[gmoss13]

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)])

[Matthijspals]

I most have introduced this, apologies will have a look!

[Matthijspals]

This should be fixed in #1185, and there is a new test that explicitly checks for this