Fix AttributeError in RAG generate() for missing config fields (#46035)

RagSequenceForGeneration.generate() crashes with AttributeError because
it accesses self.config.num_return_sequences and self.config.num_beams,
but RagConfig never defines these fields. Use getattr with a default of
1 (matching the documented defaults) instead of direct attribute access.

Also removes stale references to examples/rag/use_own_knowledge_dataset.py
which no longer exists in the repo.

Fixes #46015