Automatically infer size of subplots from InferenceData object

[Silviculturalist]

This follows up on #1440 .

Would it be possible to change the warning from az.plot_trace such that it refers to the total number of subplots, not the number of rows?

Best regards

[OriolAbril]

Yes, a PR to fix how plots are counted in plot_trace would be welcome, not sure we'll be able to get to it anytime soon. Something in this code block needs fixing: https://github.com/arviz-devs/arviz/blob/main/arviz/plots/traceplot.py#L220