Right now, the predictive distribution of GPs is sampled using `sample_ppc`

, since the conditional distribution is known given the samples from the gp. This is kind of awkward, and not done for other PyMC3 models. It’s awkward because the `.conditional`

distribution has to be added to the model after the inference step, e.g.

```
with pm.Model() as marginal_gp_model:
cov_func = pm.gp.cov.ExpQuad(1, lengthscales=1.0)
gp = pm.gp.Marginal(cov_func=cov_func)
sigma = pm.HalfNormal("sigma", sd=5)
y_ = gp.marginal_likelihood("y", X=X, y=y, noise=sigma)
# inference step here
with model:
trace = pm.sample(1000)
# then add conditional to model
with model:
f_star = gp.conditional("f_star", Xnew=Xnew)
# then sample without NUTS using sample_ppc
with model:
pred_samples = pm.sample_ppc(trace, vars=[f_star])
```

I’ve been trying to think of more PyMC3-ic ways to do this, without sampling the conditional using NUTS, which greatly slows down inference. Here are some thoughts:

- Use an empty shared variable for
`Xnew`

, such that the covariance matrix of conditional will be empty. This errors of course. It would work kind of like how prediction with regression models works. - A Gibbs step method, so that
`.conditional`

is sampled directly, or a step method that just skips the`.conditional`

RV when it is assigned to it.

Sampling from the GP conditional can be really slow, so its good to be able to completely skip it while NUTS is run. Any ideas or preferences? Or is it fine how it is?