Add `model_to_minibatch` transformation to convert all `pm.Data` to `pm.Minibatch`

[jessegrabowski]

Description

A pain point for me when testing different algorithms (e.g. MCMC vs VI) is that I don't want to write a 2nd version of the model with pm.Minibatch on the data.

This PR adds a model transformation that does that for the user. It's the reverse of the remove_minibatched_nodes transformer that @zaxtax implemented recently.

This is a WIP, it doesn't actually work now, because I can't figure out how to rebuild the observed variable with the total_size set correctly. Help wanted.

Related Issue

• [ ] Closes #
• [ ] Related to #

Checklist

• [ ] Checked that the pre-commit linting/style checks pass
• [ ] Included tests that prove the fix is effective or that the new feature works
• [ ] Added necessary documentation (docstrings and/or example notebooks)
• [ ] 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--7785.org.readthedocs.build/en/7785/