Fix retriever SDG dedup plugin circular import

[oliverholworthy]

Fixes the error printed in embed receipe sdg stage:

Failed to load plugin from entry point 'data-designer-retrieval-dedup': partially initialized module 'retriever_sdg.deduplication' has no attribute 'dd_retrieval_dedup_plugin' (most likely due to a circular import)

This error didn't prevent the stage from running sucessfully, but is a confusing error to show up in the logs.

• Avoid a circular import in retriever_sdg.deduplication by publishing the dd_retrieval_dedup_plugin before importing ColumnGeneratorCellByCell, so Data Designer's re-entrant plugin discovery can resolve the plugin while the generator base class import is still in progress.
• Add __all__ to make the public surface explicit.

Test plan

• nemotron embedding SDG dedup stage loads without ImportError on plugin discovery
• Run the retriever SDG dedup step end-to-end against a small sample

[shan-nvidia]

lgtm.