Add tests to ppe

[rohanbabbar04]

Closes #433

Description

• Add tests for ppe.
• Return new_priors in ppe.
• Add random_state parameter to get a unique answer.

Checklist

• Code style is correct (follows pylint and black guidelines)
• Includes new or updated tests to cover the new feature

[rohanbabbar04]

Hi @aloctavodia ,

I think we can merge this PR, with the two tests and slight tweaks. I have opened a new issue #437 , I will be modifying the code and adding the tests for hierarchical models there itself(opening a new PR).

[aloctavodia]

Better to add the test now. Just write separate tests for these 3 models. The other hierarchical model can wait.

normal_normal

with pm.Model() as model:
x = pm.Normal("x", mu=0, sigma=10)
z = pm.HalfNormal("z", 10)
y = pm.Normal("y", x, z, observed=Y)

target = pz.Normal(mu=174, sigma=20)

norma_normal_shape

with pm.Model() as model:
x = pm.Normal("x", mu=[0, 1], sigma=[10,10]) # This has shape 2
z = pm.HalfNormal("z", 10)
y = pm.Normal("y", x[idx], z, observed=Y)

target = pz.Normal(mu=174, sigma=20)

hierarchical

with pm.Model(coords=coords) as model:
μ_mu = pm.Normal('μ_mu', mu=0, sigma=10)
μ_sd = pm.HalfNormal('μ_sd', 10)
σ = pm.HalfNormal('σ', sigma=10)

μ = pm.Normal('μ', mu=μ_mu, sigma=μ_sd, dims="aa") 

y = pm.Normal('y', mu=μ[idx], sigma=σ, observed=diff)