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Implement Gradient AllReduce

DDP's magic is one collective call — write it yourself and the magic disappears.

Key Insight​

AllReduce is the collective operation that sums a tensor across every rank and hands the result back to all of them. Doing this by hand on your gradients with torch.distributed.all_reduce reproduces exactly what DDP does automatically.

Why This Matters​

Once you can write the all-reduce yourself, DDP stops being a black box. You will understand why every GPU ends up with identical gradients, and therefore the same model, after each step.