[Ray] Improve documentation on batch inference

[richardliaw]

This commit updates the batch inference example to leverage Ray Data's new
native vLLM integration (ray.data.llm) introduced in Ray 2.44. Changes include:

• Replace custom LLMPredictor implementation with Ray Data's built-in vLLM processor
• Add configuration for continuous batching and other vLLM optimizations
• Include batch inference example in CI pipeline tests

[github-actions]

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