Approximation's sample method uses model contexts

[zaxtax]

This change makes it so calling sample on fitted approximations uses the model context or an explicitly provided model. Previously, this had a hard assumption that we only want to sample from the same model as the one we fitted.

This also deprecates self.model.

Checklist

• [x] Checked that the pre-commit linting/style checks pass
• [ ] Included tests that prove the fix is effective or that the new feature works
• [x] 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--7940.org.readthedocs.build/en/7940/