Translation from PyMC3 to PyMC

Questions v5
4

Thanks both @ricardoV94 @jessegrabowski, pytensor got me a long way. The item I am stuck with now is
1). getting samples of a particular variable from trace,
2). getting samples of a particular variable from prior/posterior predictive.

For example, the code below produces errors on the last two lines. What am I doing wrong? Here is a link to Colab with the same code for reproducibility.

import pymc as pm

n = 250
with pm.Model() as model:
    alpha = pm.Gamma('alpha', alpha=4, beta=0.5)
    beta = pm.Gamma('beta', alpha=4, beta=0.5)
    x1 = pm.Beta('x1', alpha, beta)
    x2 = pm.Beta('x2', alpha, beta)
    k1 = pm.Binomial('k1', n=n, p=x1, observed=140)
    k2 = pm.Binomial('k2', n=n, p=x2, observed=110)

with model:
    trace_prior = pm.sample_prior_predictive(500)
  
with model:
    trace_sample = pm.sample(500)

trace_prior['alpha']

trace_sample['alpha']