Fix need for dummy data in `sample_posterior_predictive`

[ricardoV94]

Description

Fix arviz backend find_constants

This also fixes a bug when doing sample_posterior_predictive with resized X data, where the old unused Y data would be retrieved and a dimension mismatch would crop up, unless a dummy Y was set prior to sampling.

Related Issue

• [ ] Closes #
• [ ] Related to #

Checklist

• [x] Checked that the pre-commit linting/style checks pass
• [x] Included tests that prove the fix is effective or that the new feature works
• [x] Added necessary documentation (docstrings and/or example notebooks)
• [x] If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [ ] New feature / enhancement
• [x] Bug fix
• [ ] Documentation
• [ ] Maintenance
• [ ] Other (please specify):

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📚 Documentation preview 📚: https://pymc--7265.org.readthedocs.build/en/7265/

[jessegrabowski]

Let's get this merged? The CI failure looks unrelated.