feat(speculative): add DFlash validation metrics

[khazic]

What does this PR do ?

Unify DFlash, Domino, and JetSpec validation metrics across distributed ranks and add optional W&B logging for training and validation.

Changelog

• Add token-weighted validation loss and accuracy using globally reduced additive statistics.
• Add block-weighted acceptance length for DFlash, Domino, and JetSpec.
• Add Domino final-head and base-head validation diagnostics.
• Add optional W&B configuration and train/val metric logging.
• Document the metric keys and example W&B configuration.
• Add unit coverage for weighted aggregation, short batches, W&B logging, and rank-symmetric multi-process reduction.

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Additional Information

• Related to [Tracking] Speculative decoding: implemented features and remaining gaps #2958
• Tests: 797 passed, 12 skipped.
• YAML validation: 436 example files linted successfully.
• Formatting and linting: ruff passed.

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