Add a function to create a tensor that represents the loop over the posterior

[lucianopaz]

Description

This PR adds the loop_over_posterior function. It complements vectorize_over_posterior and can be used whenever vectorize_graph is not implemented for some nodes in the computational graph (i.e. some kinds of AdvancedSetSubtensor raise NotImplementedError when trying to vectorize them).

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

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

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📚 Documentation preview 📚: https://pymc--7885.org.readthedocs.build/en/7885/