[AIR][Docs] Remove BatchPredictor from examples (#37178)

[bveeramani]

The following examples already use updated APIs:

• Stable Diffusion Batch Prediction with Ray AIR
• GPT-J-6B Batch Prediction with Ray AIR (LLM)

The following examples have been updated to use updated APIs:

• Training a model with distributed XGBoost
• Training a model with distributed LightGBM

I've removed batch prediction sections from the other examples, and, where appropriate, linked to the batch inference user guide.

Why are these changes needed?

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