Metadata.to_dict fails on metadata instances pre 1.9.0

[amontanez24]

Environment Details

Please indicate the following details about the environment in which you found the bug:

• SDV version: 1.9.0
• Python version: Any
• Operating System: Any

Error Description

If you saved a synthesizer on a version of sdv < 1.9.0, and then you load it and call

metadata.to_dict

it will crash with the following error

  File "/usr/local/lib/python3.11/site-packages/sdv/metadata/single_table.py", line 295, in to_dict
    value = getattr(self, f'{key}') if key != 'METADATA_SPEC_VERSION' else self._version
            ^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'SingleTableMetadata' object has no attribute 'column_relationships'

This is because of the following line:
https://github.com/sdv-dev/SDV/blob/6f8f50b4b8d722d67b7da03fa3e75344f3f03a9f/sdv/metadata/single_table.py#L295C26-L295C26
We should handle the case where the attribute is not on the metadata class more gracefully.

Steps to reproduce

• Load a synthesizer that was created in a version of sdv < 1.9.0
• get its metadata
• call metadata.to_dict