VAE: Improved ELBO plots

[martinrohbeck]

Hey there,

this PR corrects and improves the plots in the Pyro VAE tutorial.

There seem to be three minor hiccups in the code:

• It looks like you are plotting the Test ELBO but with training epochs on the x-axis, here
• The size argument here does not exist in the current seaborn version anymore and raises an error.
• Currently both train and test ELBOs are passed to plot_llk, but only the test ELBO values are plotted. I guess this is not intended.

The PR plots both train and test performance and ensures ELBOs are correctly aligned with the epochs (even if ELBOs are computed at different frequencies) and moves the plotting function call into the if statement when the data is actually evaluated on the test data.

I did not open a PR because this is a minor correction -- hope that's fine with you.

New example plot: