Rename IndependentSynthesizer to ColumnSynthesizer

[npatki]

Problem Description

Currently SDGym exposes a single table IndependentSynthesizer that is meant to synthesize columns independently (i.e. it does not learn any correlations). This is meant to be used only as a baseline for other SDV synthesizers.

However, in the SDV library we also have a multi-table IndependentSynthesizer that is mean to synthesize tables independently (i.e. it creates random connections between tables). This synthesizer does learn the correlations within each individual table.

This can create confusion because even though the synthesizers have the same name, they work very differently.

Expected behavior

Rename the SDGym synthesizer to ColumnSynthesizer to denote that it synthesizes one column at a type, without considering correlations.

(We can leave the multi-table SDV one as-is.)