Allow opting out of model nesting by setting `model=None`

[ricardoV94]

Description

This PR allows setting model=None to explicitly disable model nesting. This is needed for model transformations do, observe, clone, remove_value_transforms, freeze_rv_and_dims to be possible under a model context without side-effects on the original model.

I still think model nesting is an overkill, but a lot of code out there depends on it so I am not touching it.

I also updated the docstrings with more relevant (imo) examples. The previous example was broken as per #6715 and I am not sure we really want to incentivize people to subclass PyMC models. That certainly shouldn't be the focus of the docstrings?

Finally, removed deprecated functionality.

Related Issue

• [x] Closes #6715
• [x] Related to #7268
• [x] Related to https://github.com/pymc-devs/pymc-experimental/pull/345#discussion_r1627311093
• [x] Related to #7348

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
• [x] Documentation
• [ ] Maintenance
• [ ] Other (please specify):

---

📚 Documentation preview 📚: https://pymc--7352.org.readthedocs.build/en/7352/

[ricardoV94]

I wonder how .to_graphviz looks for hierarchical models 🤔

Doesn't do anything different for the outer model. You can just call .to_graphviz on submodels and will only show the variables that belong to it IIRC