Improve example on expanded sample_posterior_predictive

[ricardoV94]

Description

Sample posterior_predictive relies on other groups to infer volatility, so it's important to keep the whole InferenceData.

Also there's a shape error if sample_dims is not specified

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

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

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📚 Documentation preview 📚: https://pymc--7222.org.readthedocs.build/en/7222/