Fix/llm bugs empty extraction

[AlonsoGuevara]

Description

• Add a singleton to manage llm instance creation
• Error out properly when either entities or relationships fail to be extracted

Related Issues

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Proposed Changes

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Checklist

• I have tested these changes locally.
• I have reviewed the code changes.
• I have updated the documentation (if necessary).
• I have added appropriate unit tests (if applicable).

Additional Notes

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