fix: allow same vector_id in different indexes for SQL-based Document stores

[anakin87]

Related Issues

• fixes Using multiple indices in FAISS seems broken #3327

Proposed Changes:

In SQLDocumentStore:

• vector_id is no longer unique
• added a UniqueConstraint for index + vector_id

In FAISSDocumentStore:

• modified update_embeddings method to take this change into account.
  Now for every index, vector_id starts at 0

How did you test it?

• As suggested by @ZanSara, I locally ran the tests using PostgreSQL with pytest . --document_store_type="sql,faiss"
• Added new tests for SQLDocumentStore and FAISSDocumentStore

Notes for the reviewer

• Tests can be improved/redesigned. I am open to suggestions.
• New test in test_document_store.py will probably not run until ci: Add workflow for tests using InMemoryDocStore and SQLDocStore #3332 is merged
• Fixed other little things here and there

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added tests that demonstrate the correct behavior of the change
• I've used the conventional commit convention for my PR title
• I documented my code
• I ran pre-commit hooks and fixed any issue

[ZanSara]

Spot on! No remarks, this is good to go right away. Thank you! 😊