Beta parameter is ignored in GPVAE when using the IWAE setting

[AlexandreAbraham]

Issue description

In the backbone of GPVAE, a different path is taken depending on whether IWAE is used or not.

In the case K=1, the elbo is updated like this:

elbo = -nll - self.beta * kl

In the case K>1, the elbo is updated like that:

elbo = -nll - kl

I understand that those usages are different, but it feels wrong to me to silently ignore a parameter. Also, I do not see why beta could not be applied in the IWAE case. I therefore suggest one of:

• Raise a warning if K is set >1 and Beta is specified by the user to avoid misunderstanding
• Change the IWAE loss to be computer as the beta-VAE one.

Let me know so I can propose a PR.

Thanks for the great software!

[github-actions]

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