Fix ax argumentin plot_elpd

[agustinaarroyuelo]

Description

plot_elpd can now take the ax argument. Naive example:

d1 = az.load_arviz_data("centered_eight")
d2 = az.load_arviz_data("non_centered_eight")

fig, ax = plt.subplots(1, 2, figsize=(10, 4))

az.plot_elpd({"Centered eight": d1, "Non centered eight": d2}, xlabels=True, ax=ax[0])
az.plot_elpd({"Centered eight": d1, "Non centered eight": d2}, xlabels=True, ax=ax[1])

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[OriolAbril]

Good catch, thanks! the same thing is also needed in line 131 in the case of >2 models being compared, where the ax is an array like in pairplot

[OriolAbril]

I think it should be ax.ravel()[0] or something like this, because in these cases ax is an array of axes, they should all have the same figure as reference but the array does not know about the figure.