Add deterministic DDP training example with hashed seeding

[farukalpay]

Summary

• add ddp_train.py example demonstrating deterministic DDP training on CIFAR-10
• implement SHA-256 based subseed generation with Philox generators for unique RNG streams per rank/epoch/worker
• print per-rank first batch labels and initial conv weights, plus final model_init subseed and digest

Testing

• pre-commit run --files examples/ddp_train.py (hooks skipped: no files to check)
• torchrun --standalone --nnodes=1 --nproc-per-node=2 examples/ddp_train.py --master-seed 314159 --run-id R42 --epochs 1 --batch-size 16

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https://chatgpt.com/codex/tasks/task_e_68ac7ce9b2b88323b2a9f5ba870b9768