Don't crash with ZeroDivisionError if initial test ELBO is the best test ELBO

[alecw]

The --final-elbo-fail-fraction test checks that the final test ELBO isn't much worse than the best test ELBO. It checks the (distance between final test ELBO and best test ELBO)/(distance between initial test ELBO and best test ELBO).

This causes a ZeroDivisionError if best test ELBO == initial test ELBO.

Solution:
If --final-elbo-fail-fraction is enabled && final test ELBO < best test ELBO && initial test ELBO == best test ELBO
then training is considered to have failed.

[sjfleming]

Hi @alecw , any chance you could rebase this on top of dev post #345 ? I think the test_elbo referred to here should now probably be model.loss['test']['elbo']

[alecw]

Hi Stephen, yes, fixed conflicts, addressed removed local variable and rebased. thanks!

[sjfleming]

Thanks so much!