Keep 0 dimensional parameters as 0 dimensional instead of 1 dimensional

[tomicapretto]

Until now, we've been treating 0-dimensional parameters (a slope parameter for a 1d numerical predictor) as 1-dimensional. It was not creating any visible issues until we have #572.

This PR creates 0-dimensional parameters in PyMC when they're 0-dimensional so we avoid unnecessary dimensions in the resulting InferenceData object.

Following the example in the issue, now it works.

import bambi as bmb
import pandas as pd

df = pd.DataFrame({'x': [1,2,3], 'y': [2, 3, 4], 'z': [2, 2, 4]})

test_model = bmb.Model('y ~ 1 + x', df)
test_model.build()
test_model.graph()

test_approx = test_model.fit(inference_method='vi')
test_tr = test_approx.sample(draws=1000)
test_model.predict(idata=test_tr, data=pd.DataFrame({'x': [4, 5], 'y': [1,1]}))

Also with two predictors

test_model = bmb.Model('y ~ 1 + x + z', df)
test_model.build()
test_model.graph()

test_approx = test_model.fit(inference_method='vi')
test_tr = test_approx.sample(draws=1000)
test_model.predict(idata=test_tr, data=pd.DataFrame({'x': [4, 5], 'y': [1, 1], 'z': [3, 3]}))

[codecov-commenter]

Codecov Report

Merging #575 (bc1462c) into main (32525a8) will increase coverage by 1.12%.
The diff coverage is 92.23%.

@@            Coverage Diff             @@
##             main     #575      +/-   ##
==========================================
+ Coverage   86.02%   87.15%   +1.12%     
==========================================
  Files          32       29       -3     
  Lines        2691     2413     -278     
==========================================
- Hits         2315     2103     -212     
+ Misses        376      310      -66     

Impacted Files	Coverage Δ
bambi/backend/pymc.py	82.18% <ø> (ø)
bambi/families/univariate.py	87.20% <75.75%> (+0.61%)	⬆️
bambi/backend/terms.py	96.29% <100.00%> (+0.04%)	⬆️
bambi/families/link.py	65.71% <100.00%> (ø)
bambi/families/multivariate.py	97.67% <100.00%> (+0.23%)	⬆️
bambi/models.py	85.63% <100.00%> (+0.37%)	⬆️
bambi/utils.py
bambi/terms.py
bambi/__init__.py

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[tomicapretto]

It's much easier to review this one. We can merge this right after we merge #573