BUG: Remove x_validate and y_validate as they're unnecessary

[blkboxeconomist]

This PR removes unused validation data parameters (x_validate and y_validate) from the train_jax_model function signature. These parameters were never referenced in the function body, as validation is performed after training completes rather than during the training loop.

• Removes unused x_validate and y_validate parameters from train_jax_model function signature
• Updates both function calls (warmup and timed runs) to match the new signature

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[Copilot]

Pull request overview

This PR removes unused validation data parameters (x_validate and y_validate) from the train_jax_model function signature. These parameters were never referenced in the function body, as validation is performed after training completes rather than during the training loop.

• Removes unused x_validate and y_validate parameters from train_jax_model function signature
• Updates both function calls (warmup and timed runs) to match the new signature

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[mmcky]

thanks @blkboxeconomist