viable/strict/1792521211: [funcol] Add broadcast autograd (#200882)

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_c10d_functional.broadcast has no autograd, unlike torch.distributed.nn.functional.broadcast . Fixes #200877 . Backward sums the grads onto src with the functional reduce, which returns zeros off src. This keeps backward rank agnostic, so it traces the same graph on every rank. Test plan: python -m pytest test/distributed/test_functional_differentials.py This PR was authored with an AI assistant (Claude). Pull Reque…

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