Passing only free_RVs to `sample_prior_predictive`

[williambdean]

Passing only free_RVs to sample_posterior_predictive will cause errors due to logic assuming the prior_predictive group.

Should I be able to do this?

mmm = MMM(...)

mmm.sample_prior_predictive(X, var_names=["saturation_beta", "adstock_alpha"])

If not, it doesn't fail automatically which I think should be forced if that is a requirement.

Failure reasons:

Version is trying to be attached to the prior_predictive (which wouldn't be present)

pymc-marketing/pymc_marketing/model_builder.py

Line 879 in 8475c1c

prior_pred["prior_predictive"].attrs["pymc_marketing_version"] = __version__

The prior_predictive group is explicitly being extracted

pymc-marketing/pymc_marketing/model_builder.py

Lines 888 to 890 in 8475c1c

	prior_predictive_samples = az.extract(
	prior_pred, "prior_predictive", combined=combined
	)

[williambdean]

Any idea on the behavior here. Should the prior predictive be supported without the target included?

Thoughts @juanitorduz

[juanitorduz]

Should the prior predictive be supported without the target included?

I think we can restrict it to ensure the consistency of the results.